I'm looking through my /users/{id}/reputation and I think I see a possible bug regarding how positive_rep is calculated when someone upvotes your question.
http://api.stackoverflow.com/0.8/users/40906/reputation
I have 10 positive rep for my C# question (on_date=1273991678). Looking through my reputation history on the SO site, I see that I only had 1 upvote for that question in the last month.
That leads me to believe that the positive_rep should be 5, and not 10, since I only actually received 5 rep.