Lanny Davis says so:
Take Rev. Wright, whose association had the potential to be very damaging to Mr. Obama. The early airing of this issue challenged Mr. Obama to deal with it directly. He first made an eloquent speech in Philadelphia on the issue. Then, after Rev. Wright's ill-timed and over-the-top appearance at the National Press Club in Washington, Mr. Obama decided to repudiate him directly and withdraw from his church.
Is there any doubt that Mr. Obama is better off because all of this happened last spring rather than having to deal with it for the first time in September or October? Yet go back to the videotape, and see how many pundits you can find denouncing Mrs. Clinton for expressing concerns about the Wright issue. Where are they now?
I think there's something to this. (Also, remember how shocking it seemed when George Stephanopoulos first raised Ayers at that infamous ABC debate? Now he's pretty old news--though admittedly less so for many Americans who might not have followed the primaries closely.) I've also heard Obama people say that the state-level ground operations they built during the primaries, in a place like Nevada for instance, are coming in very handy today.