This is a good question about how to prove citizenship. I do have a passport but my wife only got one when we decided we wanted to go on a trip outside of the states.
Maybe they could have a special mark on the driver license indicating US citizen thus allowing resident aliens to have a license as well.
I wonder if this could lead to a US citizen ID card at some point???
By federal law after 08 you won't be able to get on an airplane or enter a federal building without a certified drivers license or papers showing that you are a legal resident.
The DMV will still issue ID cards to those without a license but even those will only be issued to legal residents or citizens.
Without an armload of phony papers that can't be checked as invalid an illegal won't be able to get one.
One can only hope.