I noticed on a few commercial sites (I believe it was an Anheuser-Busch site) that asked for your name and city, state (and possibly email for unique identifier?). Then, it magically age verified you. It was pretty nifty.
I believe they must have used a product like ChoicePoint's Age Verification (http://www.choicepoint.com/business/financial/ageverif_fs.html). Basically a database of public records.
Hope that helps, Wayne Chang