I installed iTunes 9 about an hour ago and I've been playing with that. Unfortunately, iPhone 3.1 is required to be able to sort apps in iTunes. (At least, I can't figure out how to do it with iPhone 3.0.) So, I'm installing 3.1 on my iPhone 3GS right now.
I've had tethering working since 3.0 was first released. Tethering continued to work through several iTunes updates and minor iPhone firmware updates. I activated tethering by hacking iTunes to allow me to manually install the appropriate ATT .ipcc file way back when, whatever that method is called.
Anyway, I'll know in a few minutes whether 3.1 breaks tethering for me. I'm also on AT&T and in California.
BTW, if I lose it, no big deal. I did it more out of a curiosity than anything else. I bet I haven't used it more than 5 minutes worth in 4 months.
Mark