As far as I'm aware you can't (yet). It would make sense that they show the "blurb" that is shown on the networks during the week prior though. You can preview music videos though.
I sent Apple an email from iTunes about this issue. Steve did say that you could preview TV show episodes to make sure it was the one you wanted but I have yet to be able to do this.
On another note, I wanted to download the Thriller video yesterday. I went to the music video area and choose browse->all and scrolled down to Michael Jackson. When I clicked on his name only two music videos were listed! I then went back to the main page where there was a link advertising his entire video catalogue and sure enoug it was there. I wonder how many more are like that in the browse function. They could miss out on sales if they aren't listed. Also, I am unable to search for music videos using the search function. Only music files show up.