RealClearPolitics has a great summary of major polls over the past year that show the remarkable and recent Obama surge among Democratic voters:
The graph results seem to suggest that at about the time of Obama's strong Super Tuesday showing, Democrats stopped simply assuming Hilary Clinton would be the nominee and gave careful consideration to Obama who they found more appealing. Given Clinton's consistent showing in polls of some 47% probably Clinton simply didn't gain any converts, while Obama has pulled in the undecideds and shifted a few making him the presumptive nominee.
One of the many great challenges of the American Democracy process is the fact that in one sense it is the undecided voters that ultimately make the decisions. Because voting can't assign extra points for being better informed or caring "more" than your neighbor, votes all count equally (Democrat superdelegates aside!). Thus those who have only marginal interest in the outcome and are easily swayed by campaign tactics are a key voting block, and may ultimately hold the key to success in this presidential election.