Just came back from Roanoke a few weeks ago (scouting it out as a potential place to move - let me know what you think). Didn't have time to do mountain biking, so I took my fixed gear road bike. Anyway, found lots of excellent info for both trails and road rides here:
http://www1.roanoke.com/outdoors/biking/
Here's a link for the local bicycle club
http://www.blueridgebicycleclub.com/
Looks like there are lots of great trails, it killed me not to take the MTB, but that just means I'll have to go back.