It rankles many (I think I'm actually more rankled by Huckabee than McCain, but I digress), but what happens to Huckabee now? His gains will be dwarfed by McCain's and Romney's in winner-take-all states, but the gains are substantial enough and in an important enough region and demographic to warrant some serious consideration about where he goes from here if he doesn't win the nomination.
Matt mentioned the Huckabee vice-presidency for McCain (or Romney). That makes some more sense for Romney, I think, although the two of them don't care much for each other. For McCain, doesn't the Huckabee vice-presidency knock the legs out from under all his independent support? Although it would seem to be a decent choice on paper since he's Southern and evangelical, every moderate/libertarian Republican or Independent I know would seriously consider sitting out instead of voting for a ticket with Huckabee on it.
The thing about McCain, though, is he has this self-destructive political tick, which would not surprise me if it kicked in and compelled him to pick Huckabee as his veep. After all, Mac likes Huck, and in Mac's mind, that's usually reason enough to make a huge decision. After all, "politically unpopular" is akin to validation for every decision for McCain isn't it?
And, if Huckabee doesn't get a veep slot, does it tick off evangelicals enough to sit home? The party surely can't afford to spurn a guy with this much natural support in such an important constituency too easily. There are reasons people are for him, even though I don't agree with them.