Have you tried to activate your iPhone to no avail? Are you holding a $299 brick in your hand? Here's what you do. When activating your iPhone (or just upgrading to 2.0 software), if you get an error message on your phone, DO NOT UNPLUG IT. That will restart the entire process.
Instead, just wait. Leave your phone be and don't close iTunes.
We've successfully activated five different iPhone 3Gs with error screens just by letting them sit plugged in for anywhere from 10 to 15 minutes. Hopefully that might work for you, too.