your in luck, and you dont even have to download/buy an ap. I have been using this site: 3click.tv which fully supports streaming their library of tv series which is now something over 30,000 episodes on the iPhone. I dont know how they do it but their files are mp4 and work exactly the same on my PC as they do on my iPhone so it must be Flash video. The resolution on the phone is fantastic and pretty good on my laptop too. It does not work on a 3g connection, but I usually use Wifi to keep my data costs down anyway. With a good solid connection you looking at buffer free TV on your phone (I noticed they are utilizing progressive streaming, meaning if you do have a week signal, pause it and let it load up but it is rare in most environments as it streams very smoothly.)
All their stuff is ad free and each series has complete seasons so I like it. It is also rumured that Hulu.com has been trying to work in conjunction with Apple to get their programming on the phone in the future. Anyway, good luck with 3click.tv - it is by far the best thing I have found to do with my iPhone