Good item from Steve Kornacki (who has been doing great work on GOP WH 2012, which I feel that I haven't linked to enough; you should be reading him) about what's at stake in South Carolina. He's right: if Romney does well there, he'll be in strong enough shape that the party will begin to act as if the nomination has been clinched, which will in turn mean that he can begin his general election campaign. In my view, he'll in fact lock up the nomination even if he loses to Newt Gingrich there, but Newt will be much more of a nuisance for a while if South Carolina goes his way. In particular, Romney would certainly have to campaign in Florida as if the nomination was in doubt -- and there's a good chance that that he'll also have to take Super Tuesday seriously.
We don't really have a name for that phase of the process: the point at which the last serious threat to win the nomination is gone, but others are still campaigning and could even win some states. We probably reached that point in 2000 for George W. Bush after Iowa; I'd argue that Bill Clinton achieved that in New Hampshire in 1992, probably. It may be the case that Romney really hit that by crushing Rick Perry in Iowa this year, but he'll certainly do it in South Carolina if he knocks Perry and Rick Santorum out.
Note that it's also quite possible that a solid Romney win on Saturday leads to a rapid end to every campaign but that of Ron Paul. It wouldn't be a shock if a Romney win is followed rapidly by a ton of endorsements, along with everyone but Paul dropping out by the end of the week. If the polling is even close, Saturday will mark the end of any realistic uncertainty about the Republican nominee. But there still are lots of possible endgames, or maybe the better term is post-endgames; we'll have to see whether Romney has to pretend the nomination is still in doubt or not.