While Clinton still leads in all the polls of likely Democratic voters, Obama has jumped into second place in the race without much effort. Granted, his book tour helped fuel speculation he would run, which he did not deny (more salesmanship than ambition, I think). But now Hillary is standing in Obama's shadow at this point -- not where she wants to be.
The trip to New Hampshire next week changes everything for Clinton. It tips Obama's hand to an extent. And with the popular support he has (more like Bill Clinton than Hillary will ever be), she has to rethink her plans. Some believe she won't do anything different because of Obama -- I think that would be a mistake. But then I'm just a blogger.
I think that if Obama wants to run, he will easily catch and surpass her in popularity -- and fundraising. He is already a party leader sought by the media for comment, and he is able to raise enormous sums of money for candidates. Add to that the perception that he has heart and integrity (see Tom's post above), something most people would never say about Clinton, and he has a real shot at the nomination -- if he wants it.
You can compare their records here. So far, they are pretty close, but I think Obama is more liberal without alienating the faith-based crowd, and Clinton is trying to be more centrist (which feels like playing against type).