Streaming a movie to my iPhone 3GS through ITunes
With iOS 4.3 you can enable home sharing while you are in the same wifi network. First make sure home sharing is running in iTunes. Then on your iPhone go to Settings - iPod, and then enter your home sharing account info. 

Then, when you go to the iPod app, you can select the shared library. 

As long as these videos are encoded the correct way, they should play through home sharing by streaming to your phone. I have tried this with music and movies on my iPhone and iPad.