A couple of things to think about when considering whether McCain's poised to run away with the GOP nomination:
1. Yes, McCain won last night in South Carolina, but it wasn't a huge victory -- 33% to 30%. A higher-than-expected showing for ole Fred -- 16%, where he was polling 11-12% in the days beforehand -- can account for a lot of Huckabee's deficit. I don't see McCain's Southern win as so resounding that it proves rank-and-file conservatives are ready to back him en masse.
2. A big, and unique, factor in McCain's win was South Carolina's own psychodrama with him. In McCain, South Carolina conservatives who are fed up with Bush have an in-the-flesh way to redeem the mistake of electing him: Vote for McCain, the guy they destroyed to make room for Bush in 2000, and, along the way, atone for some of the leftover guilt from the slimy, humiliating way he was defeated in the state. McCain's campaign itself played up this dynamic, hawking repentant former Bush supporters at its rallies. "He deserves it," was a phrase I heard a whole lot from McCain supporters when I was in South Carolina, which was as much a specific reference to 2000 as it was some more generalized feeling of admiration, like the sense that Martin Scorsese deserved an Oscar.
I don't see "he deserves it" being on the lips of as many Floridians, Georgians, Californians, and so on -- they're starting more from scratch, and don't still gossip about the drama of 2000.
P.S. Commenter Brent points out an interesting statistic, via The Fix, that seems to undermine the idea that McCain won over a lot of voters this time around that he lost in 2000. From The Fix:
When McCain lost to then Texas Gov. George W. Bush in the 2000 South Carolina primary, he received 42 percent of the vote and roughly 237,000 votes. It was painted as a crushing loss. Eight years later McCain took 33 percent of the vote and 143,000 votes. It was cast as a triumphant win. .... [E]ven if McCain had pulled in only 75 percent of those voters who supported him in 2000, he would have neared 40 percent of the vote share this time around.
-- Eve Fairbanks