I'm sure it'll happen eventually. (To the person above me, I think you misread the OP. It says nothing about cashing in fairly, in fact it says nothing about cashing in at all.) MITB holders have gotten title shots in the past. Or maybe the reverse happens, and one of the champions challenges for the MITB (since they can technically be defended, although that hasn't happened nearly as much as it did in the early MITB days.) and wins it.
That could make for a good feud, actually. And it could happen right now, since Punk and Ziggler have been having issues with each other. Ziggler wants a title shot, Punk tells him to put his MITB on the line to earn it. Winner walks away with the WWE Championship and the MITB to the World Heavyweight Championship. Obviously there's a lot of directions you could go from there, although the fact that The Rock is already lined up for a WWE Championship match(and, presumably, a victory) at Royal Rumble kind of stifles their choices a bit.
I think the only other obvious way to unify the titles would be to have the MITB holder win the Royal Rumble, cash in on one title, and choose to challenge for the other at WrestleMania. The important thing is that if they do unify the belts, they need to be maintained as separate entities(defended separately, etc) to make it easier to split them up when the time comes.