The only one I know of would be Byron, which has been discussed here fairly recently: soakersbible.tribe.net/thread...c91675c

I spent a lot of time up there growing up (or failing to grow up) and never heard of it, we mostly hiked on the West side canyons but I also did a lot of reading and dirtbag research on the mountain and never noticed anything there either. We did find an interesting cold water spring out on the southern slope behind blackhawk,,,,tell the guard you are going up to see John Madden if he still lives there and they let you in. The spring was up on the shoulder of the mountain in a large area of crumbly boulders we were looking at for climbing. I remember it had cut a narrow trough through the sandstone bedrock which had turned bright blue from minerals in the water. It was an old burned area with downed pines lying so you didnt even see the ground a lot of the time hiking. Might be in there somewhere.
Curious to hear if something is there.
m

I can confirm thermal activity in Walnut Creek, as can anyone who's driven on Ygnacio Valley Road where it passes John Muir Hospital with the windows down...the sulfur smell is amazing (great spot for a hospital...)! Even with the windows up it's pretty stinky. If you're in the car with a passenger who's unaware of the geothermal activity they'll probably look at you funny as you drive through there.

I looked up the spring on soak.net - it's in there as "Sulpher Spring", hot spring number 341. I tried to find an outlet once, and found nothing. According to the map and the Google satellite image it should be right under the asphalt in this service yard (maps.google.com/maps
...anyway, no chance of seeing any of this spring, but yes, there is geothermal activity in the Mt. Diablo area. Maybe if they ever open up the place in Byron there'll be a place to take a dip.