Below are some good mountain bike trails in the Orange Country area:
Biking Destinations
http://www.bikingdestinations.com/lagunacoast.htm
Those of you in San Diego, there are a lot of great trails! Check out these:
San Diego Region Mountain Biking
http://www.trails.com/activity.aspx?area=14947
Another good resource for mountaing biking is this website. It offers bike reviews, forums, and other useful information.
Single Tracks
http://www.singletracks.com/