From the California Attorney General's office, but most states are like this:

What documentation is acceptable to demonstrate proof of residency for handgun purchasers?

-Utility bill from within the past three months that bears on its face the individual's name and either of the following:
http://ag.ca.gov/firearms/dlrfaqs.php

It's number 33.

From Tennessee:

Why do I have to show proof of address if the address on my identification doesn't match where I live? Why isn't a post office box good enough for an address?

-All federal firearms dealers are required to make sure that the purchaser's correct and current residence address are listed on the form. Since a post office box doesn't indicate residence, it is unacceptable for address purposes. If your primary identification does not show your current residence address, you must show proof of address through an acceptable secondary form of identification.
http://www.tbi.state.tn.us/firearm_c...arm_faqs.shtml