I think that with the scrutiny that was given by the opposing camp(s), if there was corruption it would have been found and exposed. But a quick Google finds only one possibility: Obama gets Teamster support by promising less oversight.

Of course, it isn't likely that Obama will be able to do much to reduce that oversight - so it may simply be an empty promise. And you can balance it with this promise to reduce corruption in the Federal government. (PDF) I particularly like the section to promote transparency.

I don't think it is possible to prove a conversion. But the lack of evidence of current corruption, coupled with the promise to root out corruption (and a good reason to expect experience and familiarity with corruption) makes it likely that there has been a conversion.

Of course, for anyone who dislikes Obama, this won't be proof. And I don't think there can be any proof strong enough to change such a conviction.