I was wondering this as well - with little to no discussion prior to release. As I don't have a dock to see if there is a dock mode in vanilla android - I can't say for sure whether it's there. I doubt it though as there's no app for it. I was using cardockv3 from the market and like that. Haven't tried it on the GN yet.
I just home that when the 3 pin car dock is available, not that "fake one" that verizon is currently selling.
comes out that it will kick off car mode when placed in the dock, like my droid x with its magnet.