I've had my 350z in storage with the battery disconnected. Whenever I reconnect the battery the alarm goes off immediately and I can't disable it with either key fob (I have 2). Both key fobs have new batteries. The car has been flashed with uprev so it will still start with either key, but the alarm will keep going off. I'm fairly sure if i could just stop the alarm i could do the pairing procedure again for both keys, does anyone know a way?
This is what i did to resolve this:
1) Open the door, pop the bonnet, Disconnect the battery. Technically an optional step, but it's good for your hearing...
2) Use the cable emergency release under the rear strut brace to open the boot.
3) Disconnect the alarm sounder (underneath the false floor, next to the spare tire towards the front of the car.
4) Reconnect the battery, perform the pairing procedure (see http://www.350z-uk.com/topic/18178-programming-a-key-fob/ ) for both keys.
5) Drive the car for about 30 minutes or so, then plug the alarm sounder back in. The alarm will go off.
6) Shut all doors and windows, lock the car with the fob and wait for the alarm to stop.
Everything should be back to normal now. Thanks to Ben in the comments for his suggestions :-)