The same thing happened to me this morning with my old iPhone 3G, previously synced to a PC, and my new MacBook Pro.
I plugged it in and it only started charging.
So....
First, I opened iPhoto and imported all the pictures (a prompt came up asking me if I wanted to).
Then, go in iTunes and click "File" at the top of your screen. Select transfer purchases. This will transfer everything you have bought into your Mac. Unfortunately, once you sync, all ripped CDs will be lost. I have been assured by AT&T, however, that old SMS messages and voicemails will stay.
Hope this helps