Joost has been in beta mode for almost 2 years now. Some content is pretty good and some are hard time find stuff, things that even TV wouldn't show.
Hulu.com is also beta testing right now with a lot of TV content from NBC,CBS,FOX,etc... I find that hulu tends to be more mainstream but Joost has movies that are often too hard to find on tv nowadays.
Additionally, many of the major tv studios have their own TV over IP applications direct from their site. NBC and some others.
TV over IP is definitely becoming more prevalent but the streaming quality has not yet gone to the lengths of 1080i quality. I suspect in a few years time you'll have a combo solution for Internet and TV for your living room. Heck a lot of Television manufacturers i.e Toshiba and Sony already have tvs' with wifi capability.