OP, I just tried it with mediamp4.com (once I found a video that would load) and it works fine. I've got the official Apple composite cables ($50) hooked up to the TV in my bedroom (I use it for Netflix). Any video that plays on the iPhone in the native player (not VLC) can be sent through the cables to the TV. That includes anything that will play on it from the web.
FWIW, I'm on a jailbroken phone and could have bought the cheap knock off cables but I've heard they can be quite flaky. I've yet to have any issues with the official cables.