FYI, in case anyone is interested, the car holder (the official one from VZW) for the Droid X(2) fits the GNex perfectly.
You have the take the dock apart and pull out the USB plug, else it will stick up. You also have to take out the "case insert" thing that Moto put there for DXs with cases. If you do that, then the Nexus fits perfectly, power button down.
Obviously, this does the same thing that the official VZW nexus dock does, and won't actually car dock. But if you have one lying around like I did, at least you can have the same functionality until the official dock comes out.