Funny you should ask this, I had the same problem last weekend.
I installed an amp and when I turned the car on, it asked for the code. Tore apart the manual, etc. called the dealer, late Sunday night and service was closed. Guy on the phone had no idea what I was talking about.
Was going to wait until Monday morning to call, decided to just start the car and the radio came back online with no code needed.
I should add, I never disconnected the battery, but must've lost power to the radio or something when I undid the nut on the post I was using for the amp.
If you've started the car and it is still asking for code, sorry can't help you... Post back here if you have to get it from your dealer or something. If you haven't actually started the car, try that..
Jason