I am having the same issue on both of my computers. I have 7.0.3 installed and the newest iTunes. I tried it on my iBook G4 800 and my Dual 2.0 G5, both using a 8.0 cable connection. What is the problem? Just to make sure, you just double click to preview? I don't want to miss the obvious.
The show "Lost" has an episode called "Everybody Hates Hugo" which you can preview, but none of the other episodes work. Although, to be quite honest, I'll pass on being able to watch "That's So Raven" trailers.