Obama's major problem is that he's peaking now. If he sits in the senate for another 4 years, he won't have the momentum to win the nomination. Add to that the fact that Democrats will (knock on wood) probably win the White House in '08.
There's no better time than now for Obama.
There's no other time than now for Obama.
That said, if Obama enters the race, barring Gore running, Obama becomes a big contender for the status of "anti-hillary." Kerry, Biden, and Dodd at this point have no chance.
The race comes down to Richardson, Bayh, Obama, Edwards, maybe Clark, and Hillary. One will be the nominee, and odds are one will be VP.
How it shakes out depends on what happens this November.
Democrats win back both houses, Hillary and Obama gain.
Democrats fail miserably, Bayh, Richardson and Edwards gain.
I hope Obama runs, if only to make it more dramatic