I try not to discuss DS's advanced things with the other moms unless they ask now. It just works better that way for us.
As for mainstreaming, I think it really depends on the child and what you are hoping to do. I was somewhat mainstreamed in a public school that had pullouts for math and reading. Even with the pullouts, it was too slow and I was miserable. I was also shy. I had friends but learned to essentially turn my brain off in class. My grades were good, I was seemingly well adjusted but miserable.
For my DS, I am looking for a private school for when the time comes that will have small classes and challenge him intellectually. It is not about status, but about being happy with who he is and continuing to love to learn. I have not ruled out homeschooling. This is all a bit premature though as DS is not even 2.
I think mainstreaming is less important now that technology has made the world smaller. A person can find a virtual community or hybrid community (like DYS) for peers now whereas before conforming might have been the only way to avoid loneliness. Although I would say trying to conform to have friends is really very lonely...
