Instapundit has a long post about the constitutionality of the National ID bill that is working its way through Congress (in conference right now). His point: that the federal government doesn't have the power to force the States to implement the system Congress is proposing. I had this same thought a few days ago. While I am strongly in favor of requiring people to present strong proof of identity and citizenship before obtaining a drivers license, I am skeptical that this law is within the bounds of Congressional power.
Glenn also points out that Congress would have the power to make implementation of this system a condition for receipt of federal funds. I hate this power - I think it allows Congress to ignore constitional limits on its power simply through bribery - but it's certainly constitutional.
When analyzing constitutional issues I think a lot of people fall into the trap of mixing up "constitutional" with "good policy". One does not imply the other but there's a strong tendency to uncritically conclude that a law you like is constitutional and one you don't is not, at least when it's not clear cut. I'll grant that good lawyers are less apt to make this mistake than average people, but they certainly fall victim to it as well leading to charges of judicial activism.
Everyone should be able to think of plenty of "good policies" (having desireable effects) that they realize are unconstitutional as well as the opposite. If you can't then you either don't understand the Constitution or you're not thinking critically about it.
UPDATE: I should have said "proof of legal residency" and not "citizenship" as a requirement for a drivers license. I have no problem with legal aliens obtaining a license to drive. My experiences with the horrible driving of some foreign grad students didn't affect me that much...