Jon, this is a fascinating debate, but I'm not sure I agree with you on the "substantive" point -- they've now been arguing for 10-12 minutes on whether or not it's fair to pick particular votes out of a long legislative record to attack people politically. (Well, of course it is -- some votes are a lot more important than others.)
Commenters weigh in, but I don't think Obama does well in this kind of argument -- "John, you have to agree that I didn't just follow, I led," sounded petulant. He argues Clinton on small details -- I only worked five hours for the slum-lord! -- rather than the message, and against her rhetoric it doesn't really work.
Intense line of the night, so far, from Hillary:
It is extremely difficult to have a debate with you, Senator Obama, because you never take responsibility for your votes.
-- Eve Fairbank