Here he is:
I can see why McCain brought this up now, and how he probably wanted people to find this video on the internet and see it played on cable news.
Joe looks like he's pissed off (although maybe that's just the sun in his eyes) and that he's not buying any of what Obama's selling. Obama looks like he's filibustering the guy instead of talking to him, and he gives an incomprehensible answer to Joe's suggestion of a flat tax. Obama's core philosophy, which he repeats over and over, is that people who are successful should pay more. He believes in "progressive" taxation, and that's why he's against a flat tax.
Anyway, it looks like Obama recovers pretty well at the end, and actually gets a round of applause from the crowd. But this particular video ends before the exchange is complete.
UPDATE: I replaced the NY Post video with the one from YouTube, which is more complete. Thanks, Jill.