Also, with 3G it auto-detects the speed and sends you lower quality files if needed.
And it's not just good/bad. There's a lower one for 3G and a REALLY lower one if the speed's not great. It can change in the middle of a movie sometimes if situations change.
So even besides limited data plans, I always suggest using wifi if possible for this reason.
It's possibly to hook up your iPhone to a TV, but nearly everything has Netflix in it these days. Apple TV, Roku, Playstation, Wii, many Blu Ray players, etc. I'd suggest finding one of those rather than having to use your phone to watch movies on your TV.