I got a AT&T compatible Nexus One that I'm trying to connect to my Mac Book Pro (Snow Leopard) for development. When I do an adb devices, it doesn't appear, and I can't do an adb install either. Until today, I've been using a G1 and I haven't had any issues. I haven't placed a SIM in the phone yet, but I don't think that should matter.
Ugh. Forgot to turn on USB debugging on the phone itself. It is documented. I didn't have to do this on first phone that I borrowed because it was already done.
"Turn on "USB Debugging" on your device. On the device, go to the home screen, press MENU, select Applications > Development, then enable USB debugging."