If you're interested in "extreme" mountain biking in the bay area, here are a few places to ride:
Soquel Demonstration Forest off of Summit Rd. off of HWY 17. Stop by Trail Head Cyclery http://trailheadcyclery.com/ on Union St. in San Jose to get directions, etc.
This is where you find the guys on DH bikes with body armor. Many of the trails have man made jumps & stunts ranging from small 6" log crossings to logs with 5' drops, 3' kickers, skinnys (logs) 2' up in the air, etc. The Corral Trail has great jumps, Braille has fun stunts - see-saws, logs, a few jumps, Tractor Trail has good flow, but not many stunts, and Sawpit has it all! Flow, LOTS of jumps, steep rocky sections, huge log rides, and just about anything else you could want. Definitely ride with friends at Soquel, as it's pretty remote, so if you hurt yourself it could take help a while to get there. I'd also recommend body armor for the descents.
UCSC also has fabuous riding. Again, lots of man made jumps, logs, etc. "Sweetness" and "Magic Carpet Ride" are favorites of many. Some of the trails at UCSC are legal, some aren't, and I don't know which is which. It's best to find a local to ride with, or just ask people on the trails which rides they like best.
Skeggs Point and Tamarancho also have fun single track, but neither are as technical or have as many jumps & stunts as Soquel and UCSC.