The idea of momentum is that you generate support in subsequent primaries when you win one. Not this year. Fox News anchor Brit Hume now refers to "no-mentum." Obama won Iowa, then lost New Hampshire. John McCain won New Hampshire, then lost Michigan. And so on.
Indeed, this Saturday the press expects Obama to win South Carolina because the press assumes that Hillary's "momentum" from Nevada is virtually nonexistent. Likewise, Hillary's apparent strategy--lose next Saturday, win Super Tuesday--assumes that Obama won't get any Gary-Hart like momentum from South Carolina.
Let the record show that the Death of Momentum was entirely foretold at least eight long years ago by the application of the Feiler Faster Thesis (voters comfortably process information quickly) coupled with what turned out to be the Skurnik Two-Electorate Theory (voters who don't follow politics don't tune in until the very end). As outlined in 2000, late-focusing voters tune in to what the press is saying in, say, the two days before their state's election, which is usually something different from what the press says in the two days after the previous state's election. Four days = no mo' mo. Add in possible affirmative voter rebellion against what the press says--Huck's Hot! Barack Rock Star!--and it's overdetermined. ... P.S.: Another equation--