In the past, ATF has said that you could not convert a pistol to a rifle and back to a pistol (except for the Thompson Center Kits that were sold in both configurations) without filing NFA paperwork (as a pistol made from a rifle). A new ATF Ruling (2011-4 dated 25 July 11) now says that if a firearm ORIGINALLY starts out as a pistol then it is OK to convert it to a rifle and back to a pistol. If it was ORIGINALLY a rifle, it needs to stay a rifle. This is good news for the folks who like the conversion kits for Glocks, 1911, etc.. that have a stock and 16" barrel. You can now convert them back and forth without worry.
http://www.atf.gov/regulations-rulings/rulings/atf-rulings/atf-ruling-2011-4.pdf
http://www.atf.gov/regulations-rulings/rulings/atf-rulings/atf-ruling-2011-4.pdf