In reply to gramee:
Here is a list of some places I have been and some I have heard of:
Joshua Tree - Some Sport & Bouldering
Alabama Hills - Easy/medium granite sport but still fun. This is on the road between J-tree and Bishop
Bishop - Happy and Sad boulders, and the Buttermilks for bouldering (you can drool at the huge rock routes in the high sierra as you boulder)
Owens River Gorge - its just outside bishop and has a great range of sport routes.
We were at these place in November and it was just getting to climable temps. I think September may be a little too hot.
So some places I have heard of that may be fine:
Tahoe
Trinity Aretes
Smith Rock - if you are willing to make the drive to Oregon
There are loads of 'local' venues around Cali, so I would suggest going into a proper climbing store and asking their advice. The farther north you are the better climbing temps will be. When I say it gets hot, it get scorching and September is HOT!!
Have a good trip