I came across this while doing some unrelated research on the ATF's site:
(B17) How do I obtain a classification from ATF for my "potato gun?" [Back]
Any person desiring a classification of a “potato gun,” “spud gun” or similar device must submit a written request (not e-mail) to the Director and include a complete and accurate description of the device, the name and address of the manufacturer or importer, the purpose for which it is intended, and such photographs, diagrams, or drawings as may be necessary to make a classification. A final determination may require physical examination of the device. Such requests for classification should be submitted to: Bureau of ATF, Firearms Technology Branch.
http://www.atf.gov/firearms/faq/faq2.htm#b17
What the heck is that all about? Is it a different kind of potato gun than my 11 year old son has?