I didn't think you would be able to stream from iTunes. There is an app called "Air Video" that allows you to do this. The server portion is free and the app portion (for the iPod) is $2.99. You point your server to a directory or directories of movies on your desktop - and you can stream from there. Not sure what the clarity is though - and not sure how this works on the iPad.
Netflix also has an app out that allows you to stream their "play it now" movies. I think you have to be a member.
I also heard that ABC has an app available to stream certain TV content.