Mike, I take your point about the folly of over-estimating the American voter. I think a lot of voters will bite on Hillary's gas-tax-holiday proposal. But we need to go further to figure out who wins this round.
Basically, there are two questions we need to answer here: 1.) Will Obama's position on the gas tax expand his coalition/cut into Hillary's? 2.) Will it shore up his existing coalition?
I think 1.) is an open question. I'm more optimistic than Mike for the reasons I lay out here and here, but he could easily be right. But I think 2.) is a no-brainer--among people who were already somewhat sympathetic to Obama, the gas tax debate really helps him. It reminds them what they like about Obama (he's not a panderer or a typical Washington pol) and what they dislike about Clinton (she's cynical, will do or say anything to win an election).
Going into May 6, that's a victory for Obama. If he just nails down all the voters who were leaning his way prior to Wright, he'll be tied in Indiana and up 10-15 points in North Carolina--which would be a great result for him.
Update: Don't get me wrong. I'm not saying Obama's going to fight to a draw in Indiana and win North Carolina by 10-15 points. (It's possible, but not likely.) My point is just that Obama doesn't need to win over new voters to do well on Tuesday. He just needs to hang on to all the people who were with him until about a week ago. The gas tax won't accomplish that by itself. But it's a step in the right direction.