I am a member of JK-Forum and we were in the discussion of creating a POI database for GPS Receivers that would contain a list of off-roading trails around the United States (and/or world!)
I was told this would be a good area to kick-off the idea. It seems to me this would be a very popular POI as there are lots of Off-Road Vehicles always looking for a place to drive their vehicles, as well as a large usage of GPS systems with them.
Unfortunately I know nothing about creating a POI yet.
I don't think a POI database is the way to go. POI's tend to be a list of locations, like stores, restaurants, etc, that have a physical address. Off-roading, whether by Jeep, ATV or motorcycle, lends itself more to GPS tracklogs recorded during an actual trip.
There are many websites that already offer this. The one I use is GpsXChange.com. It's mainly for motorcycles but the webmaster added an ATV section at the request of an ATV club and it's a great resource (as most of the trails are close to where I ride my off-road bike). I have found other websites that shared Jeep tracklogs but I didn't keep their address - shouldn't be too hard to find them with Google.