The chance that we could see another match between these two makes me salivate. The three matches that they put on were some of the best matches I have seen in ages. They have great chemistry and tell a great story together. To address the topic more directly:
As Dr. Awesome pointed out, the IC and (WCW) US titles were unified at another point, when Edge beat Test in '01. Edge won and was actually the US champion, not IC. Not sure how they determine which belt stays and which goes in such a case without rechristening it as something different. Anyone know? I have seen the match itself on DVD, but don't have any knowledge of how they made the determination, or rather, justified it, as obviously the intent was to get rid of the non-WWE titles.
In DBD we've got arguably one of the top five ring workers in the company and someone the crowd has really gotten behind. They sure do love their underdogs. Now, I do not wish to take a single damned thing away from him, but--strictly in terms of his recent success in WWE--he owes a lot to Michael Cole and The Miz (especially). The storyline on NXT garnered him a lot of support as someone the crowd wants to see succeed (those who didn't already know who he was, mind you). Let's face it, until the bell rings, he looks like a jobber and his mic skills are not exactly stellar. It was hard to see at the time, but pairing him with Miz was a stroke of genius. Not only did you have one of the best mic workers in wrestling being a raging dick to the guy, but you also had Cole, who to that point had been a neutral/face commentator, criticizing every aspect of DBD's character, lifestyle and claiming that he had no talent even as we were watching him display it. The result? It's like what Piper said about WM I: people weren't just watching to see Miz and Bryan, they were watching to see Bryan kick Miz's head in. Fast forward to DBD's return at SummerSlam, and the feud is back on, with Bryan ultimately winning the US title from Miz in a match that stole the show at NOC. The submissions count anywhere match elevated him further (and also made Morrison look great), and after a glorified squash match against Sheamus and a decent rematch the following week, it was the mini-feud with Ziggler that really cemented him in the card and made him a believable champion.
Not only did they have arguably THE PPV match of the year, but they went at it on Raw the next night AND that week's Smackdown. To me, this says that WWE is very high on both of these guys to the point where they wanted everyone, whether it be PPV buyers or people who prefer one show or the other or just WWE fans in general to see how awesome they were capable of being. Whether the TV bookings were a direct reaction to the PPV match or if they were planned ahead of time is anyone's guess, though. You can make a better case for people knowing who Ziggler is, given that Smackdown guys do occasionally show up on Raw, but NXT does not have high viewership, even in it's first season (it's tailed off even further since then to the point where it's not even on TV anymore), but this is what got the remainder of the crowd behind Bryan and also made him look like a believable champion, with him not only looking great in the matches, but winning all three.
Ziggler's rise has been more steady. He's paid his dues to the point of surviving The Spirit Squad and still stuck around, and has been involved in a number of high profile losses on PPV and TV in the hunt for the title over the past year and a half. He is, IMO, another of the top five ring workers in WWE, and while I do disagree with the OP that his mic work is "fantastic", he's competent and occasionally cuts a good promo (his speech on Friday about being a champion while Kofi is just a contender was a nice one). Clearly, they want this guy to do big things to the point of realizing this as his only weak spot and came up with the "relationship of convenience" angle with the perpetual heat magnet in Vicki. It's amusing that a guy who is as good looking as he is would settle for someone who isn't exactly gorgeous just because she has power and was capable of giving him an easier path to greatness. It hits home with everyone watching: this guy will do whatever it takes to win to the point of seemingly "lowering himself" romantically in order to acquire and hold onto a piece of gold. It has done great things to get him over as he has continuously taken the "cheap" route in DQ losses and countouts, pulling guys' feet off the ropes and grabbing tights to secure a pin. As a result, he has held onto the title for 4-5 months and just when you thought it was in jeopardy, he's somehow managed to hold onto it. Even with the stip in place that he'd lose the title via DQ or countout, he pulled off a clean win against Kofi. He is as believable of a heel champion as there has been in ages, and he embodies his gimmick very well in the process, both in and out of the ring.
We are at the point now where neither guy losing his respective title would bury him. Ziggler has proven that he is relevant and capable of doing big things, while Bryan has tremendous crowd support and a score of 3-0 in the previous feud with Ziggler. Obviously, booking post-loss is what ultimately makes or breaks the loser, but it doesn't seem likely that either of them would fade. Ziggler could be freed up to do other angles further up the card and DBD is always going to be able to put on outstanding matches to maintain his following and sway others toward it. While the latter isn't really going to be able to develop a feud on the mic, when he's in the ring, people don't give a shit about that: they just want to enjoy the show. The backbone of his path is as a guy who kicks ass and takes names between the ropes but comes off as an average guy who doesn't seem to have a great chance of succeeding. This is the sort of thing that the crowd loves, and he can ride that excellently for a very long time. I see Ziggler as someone capable of elevating himself much more easily and a loss would enable him to do so, backed up with the credibility of having had such a successful IC title reign, and he's perfectly capable of either staying where he is or going back down to a future mid-card title feud without losing a step. Bryan, on the other hand, has more to gain by holding onto the title, but I feel as though he's cemented his place as a prominent member of the mid-card on Raw. You could argue that Ziggler has more to gain in getting himself further bragging rights at having unified the titles, but you could also argue that Bryan has more to lose in that draping the belt over his shoulder when he's not in the ring makes an otherwise uninspiring guy look more relevant and credible. Don't forget that anyone just tuning in who doesn't know what's going on has the initial thought of "this guy is the champion, so he must be good". What could be a deal breaker is the 3-0 score. I could easily see Ziggler winning for that reason alone.
Bottom line: I don't see Bryan's luck changing without the belt, so long as they still give him TV time, and though I don't imagine Ziggler would lose ground if he lost, he's got more to gain in terms of a foundation on which to built a main event caliber future. For that reason, I'd like Ziggler to win. The feud would likely last three matches, as seems to be the trend. What could be the likeliest scenario is that Ziggler wins, maybe not cleanly, cheats to retain and screws up in a third match to bring the gold back to Bryan. At that point, Ziggler has the credentials as the guy who unified the titles, but ultimately is free to move on to bigger and better things, while a cheap loss for Bryan puts the world behind him yet again, with the ultimate result being that he comes out on top and has further justification for where he is on the card. Regardless, I'd love to see these guys wrestle each other again.
