The WWDC 2009 session videos don't play on my apple TV either. In itunes I tried right-clicking on one of the videos and selecting 'Create Apple TV Version' but itunes tells me "One or more movies were not converted because they are already in the correct format".
What I had to do to is right click on the movie in itunes and then select 'Reveal in finder'. Then, from the finder, I open the movie in Quicktime 7 and I use the export feature to export it as an apple TV movie.
I tried phoning apple about this but they were not able to help. The people I spoke to couldn't explain why it wouldn't work.