Have you looked at Telekinesis? You need a computer to be on to stream from, but it is rather close to what you're asking for -- and it's free and only limited by your hard disk space.
I regularly use it to stream things out of my iTunes library -- i.e. TV shows and movies when stuck on call at work and I blow through more than can be stored on the iPhone without reencoding.
Plus you get all of your documents too. It's pretty nifty, especially considering how quickly it's moving along.
I've even watched movies streamed on EDGE (shh, don't tell AT&T... don't think they'd appreciate that). I just turned off auto-lock and let it buffer for about 5 minutes, and after that it kept going just fine. Obviously it cries for Wi-Fi, but sometimes that's not an option.