Hopefully he does come back. He will get the biggest pop in history if he does. But the second he shows up, it's the second he has to explain why he's been gone. I don't want to believe Punk just "walked out" and "turned his back on the fans" and hopefully we get answers tomorrow night
I'm not getting my hopes up that Punk will be at Raw until Punk actually comes out the curtain, but I'll play along with this...
If Punk shows up, he doesn't actually have to explain anything. Whether the walkout is real or staged is irrelevant because CM Punk's character had a legitimate reason to leave television for the past month. He had been a bit paranoid in November and December that The Authority had it out for him - and that paranoia was confirmed at the Royal Rumble when Kane interfered and eliminated him. So the character can simply that he left because he was sick of being screwed by The Authority. Is it the best explanation? Probably not, but it would explain things regardless of how close it was to the actual truth.
As for what a Punk return would do to the main event at WrestleMania - I can't say for sure. If you're of the mindset that Punk walked out because he wouldn't be headlining WrestleMania - and thus would only return because he got that opportunity - then a fatal 4-way for the WWE Title between Orton/Batista/Punk/Bryan is possible. The reason I don't like this idea is because I fear the match would have the potential to stink up the joint - not because Orton/Punk/Bryan aren't three of the most talented wrestlers on the planet, but - because I still have memories of the WrestleMania 2000 main event that seriously stunk up the joint! (Note: Coincidentally, this train wreck WrestleMania event was actually announced at a Raw in Chicago...) This solution would also leave just 5 weeks to find a suitable opponent for Triple H, who - love him or hate him - will be in a high-profile match somewhere on the card.
Personally, I've never believed that this story was about Punk not being in the main event at WrestleMania. Punk had to see the writing on the wall months ago when it became apparent that Batista was returning and that either Cena or Orton would head into 'Mania with the title. Additionally, with Daniel Bryan's popularity sky-rocketing, there was just no chance Punk was going to main event 'Mania. Why then did it take until the end of January for him to leave?
My thought has always been that the RR crowd made the WWE realize that a Bryan/Sheamus match at 'Mania wasn't big enough for Bryan's rising star, and as such, they gave Punk's proposed match with Triple H to Bryan. I can see how that would piss off Punk - especially since a more logical solution after the Rumble was to figure a way to do a 3-way dance with Batista/Orton/Bryan. Because of that, I do believe a Punk return would call for that scenario. Bryan would be moved into the WWE Title match while Punk would be re-inserted into his originally proposed match with Triple H.
I understand why so many would hear this news and predict that Punk will be inserted into the WWE Title match with Batista and Orton while Bryan would be left to wrestle Triple H. It would be the easiest solution since Bryan and Triple H have been teasing a WrestleMania showdown since the night after the Royal Rumble. My concern with that, though, is that simply inserting Punk into the main event won't be enough to satisfy the disgruntled fans - in fact, anything short of Daniel Bryan holding up the WWE Title to end WrestleMania probably wouldn't be enough. As popular as Punk is with the crowd, there's a risk that even he could be booed for simply not being Daniel Bryan. That's why I'd run with the originally-proposed match between Triple H and Punk and move Bryan into the WWE Title match.
All this said, I'm very excited about the possibility of Punk showing up tomorrow on Raw. If it does happen, it has a chance to top all the major WWE moments that have occurred at the Rosemont Horizon over the past 30 years. These moments include the Austin/Hart I Quit Match at WrestleMania 13; Jericho's WWE debut; The Rock/Hogan showdown that led to their WrestleMania 18 match; the Punk/Cena match at MITB 2011. Call me a Punk fanboy if you want for saying that, but I do believe the Chicago crowd desperately wants Punk to be there - is prepared to revolt if he isn't there - and would absolutely explode if he was there. Than in itself would make the moment truly unforgettable if his music hit and he came out from behind the curtain.