I know this is an old topic, but, just to clarify, no jailbreak is needed if your service provider allows you to create a wifi hotspot on your iPhone (which many, if not all, do) then the Apple TV can log into this hotspot (referred to earlier in this thread as an "ad hoc" network, which it is) and allow AirPlay from any compatible device (mac, ipad, iphone) also on the network, or allow you to play streaming media accessed through the iPhone's data connection. Using the ATV to access streaming radio, for instance, is a good low-throughput option. Streaming movies from Netflix might get costly without unlimited data. If you turn on the hotspot and then just Airplay from local storage you will use almost no data. We load the iPad with several movies before a trip so that we don't have to rely on either the iPhone's data plan or tapping the shared bandwidth in a campground.
I'm investigating a software package called FireCore that works on a jailbroken ATV2 (no ATV3 support yet) and allows it to directly to a NAS (network attached storage) drive, which would be much faster to load and more capacious than the iPad. And with the prices of 2TB NAS drives plummeting it could be a pretty cheap proposition to have a few-hundred-title mobile movie library in your RV. I keep vacillating between this option and using a slingbox to access media stored on my home system (not to mention the contents of my DVR and live satellite!)... but this is a very bandwidth-dependent solution. Not that we watch much tv when camping, but it's nice to have options.