I have to agree with candycorn: I think it is very odd that Romney is challenging Reid to produce evidence. Romney could have simply (and fairly) criticized Reid for repeating unsupported hearsay. Instead he gives Reid the opportunity to simply retort with something like:
"I can't say who the person was because I would be betraying a confidence. You, on the other hand, can prove or disprove the allegations simply by releasing your own returns in accordance with tradition. You have called me a liar (since I said my source was a Bain investor and you say it was probably the White House) and you have the documents to prove your allegations. Let the American people judge which of us is the liar."
It's up to the person bringing the allegation to prove it, not the person being accused. We don't have prosecutors arresting people and telling them to prove they are innocent. The one bringing the charges has to prove they are guilty.
Otherwise we could just say that Harry Reid murdered someone when he was 20 years old. Now let him prove he never did it.