MSNBC's first read has an excellent piece on why Obama is now the front runner. Highlight:
"For Clinton to overtake Obama for the pledged delegate lead -- which we think is the single most important statistic for the superdelegates to decide their vote -- she'll have to win 55% of the remaining delegates. Assuming next week goes Obama's way in Wisconsin and Hawaii, that percentage rises to 57%. Toss in likely Obama victories in Vermont, Wyoming, Mississippi, Oregon, Montana, and South Dakota, then Clinton's percentage need tops 60% of the remaining delegates available. And this is simply for her to regain the pledged delegate lead."