Air Video can stream any movies on your computer (including in iTunes) to your iPhone, over WiFi and 3G. It'll even do real-time transcoding, so if the video is in a format the iPhone can't normally play, it'll convert it in real-time.
One thing to keep in mind is that videos purchased through the iTunes Store are DRMed, and as such 3rd party programs can't typically stream them. Air Video will setup a direct stream of the file to your phone, going through Safari. The problem with this is that the usual over-the-air encryption can't take place (because Air Video can't read the DRMed file), so your computer streams the entire file. If it's a big HD file, this will chew up a lot of bandwidth, and take a long time to start.