The federal government gets their legal authority to regulate guns, require background checks and dealer licensing through an interpretation of the Interstate Commerce Act.
As a workaround and to test the limits of this authority, the state of Montana passed the "Montana Firearms Freedom Act" in 2009, which basically exempts any gun from federal authority that is manufactured in Montana and which doesn't leave the state. In response, the ATF has said that federal law still applies and a test case is currently working it's way through the federal courts.
After Montana (according to Wikipedia), Tennessee, Utah, South Dakota, Wyoming and Idaho have all passed similar laws and just this week, the Arizona Legislature approved an identical measure, which is now on their Governor's desk.
The authors of the Arizona legislation make no bones, in either the press or in the legislative documents that their goal is to question the authority claimed by the U.S. government.
Update: Governor Brewer signed this measure into law on April 5th.
Though somewhat unrelated, but not entirely: When I was looking for the Arizona legislation, I learned that Rep. Nancy McLain (R-Bullhead City) is among the co-sponsors of a bill that would require political parties to prove the eligibility of any Presidential candidates, they forward to the state.