If done skillfully, in a debate, this truth could be used to devastating effect. Sure BO "evolved" on gay marriage and so on.
But, for the most part, he has been asking the wealthy to contribute more since, well, forever. He has been saying investemtn in education and infrastructure since, well, forever. He said full covereage for health care for since, well, forever.
Mittens can say that about nothing.
Somewhere in this is debate opportunity. Not in just the tired "flip-flopper" sense, but cleanly on each point as they occur.
"I believe that the wealthy need to pay their share. My opponent is still deciding, and a decision that might vary from state to state and week to week."
(obviously, I'm no debate coach, cause that wouldn't fly).