I'm looking to move down to Las Vegas and was wondering what kind of trails there are in that area?
As mentioned, probably the best resource for the most popular and toughest stuff is the Vegas Valley 4 Wheelers. They used to have a really good library of trail maps and descriptions on their website, but the website just underwent a major facelift and is now in a very infant state. As far as I can tell, all the maps are gone, but hopefully this is a temporary situation as they get the new website up and running smoothly. It's definitely worth looking them up once you get down there. They are a very active and heavily involved group.
http://www.vv4w.com/
As far as more difficult trails requiring modified rigs, the Logandale Trail System, a couple of desert trails near Pahrump, and several trails on the California side of Death Valley National Park are the most popular.
As far as easy to moderate backcountry exploring, there are quite a few very remote and very lonely trails on BLM land in both Nye County and in Clark County. The BLM has been less than friendly toward 4x4 and ATV access and the local governments haven't been much help. If you go down there, get involved, help fight for the trails, the off road crowd down there needs more bodies engaged in their fight.
Check out this blog regularly for updates on backcountry exploring and general land use issues in the area. Spend some time looking through the older posts and through blogger's map website.
http://offroadinghome.blogspot.com/