We are in OC with DD9. There is a meetup in north OC that I cannot speak for since I've never gone, but it apparently has 45 members:
http://www.meetup.com/Orange-County-Gifted-Parents-Meet-Up-Group/We found a gifted magnet school in a local district that clusters high achievers and gifted-identified children, which does wonders socially, but academically there is no individual differentiation. There is spelling list A or B, etc. I don't see how there can be much differentiation with 38 students in one class!
There is also a Gifted OC Yahoo! group but it's nearly dead.
We're still flirting with home schooling but DD is an extrovert who loves daily interaction with peers.
