Representative Henry Waxman (D-CA) has proposed a federal gun bill in the United States House of Representatives that would ban the sale and possession of 80% Ar-15 receivers. However, the way the legislation reads, it actually bans much more than receivers. It appears to include virtually any part used to build a semi-automatic weapon.
H.R. 2910 was introduced on August 1, 2013. The particular lower that is referenced is one that is not considered a firearm because it still requires some basic machine work before it is ready to be used in building a firearm. The benefit to purchasing an 80% lower versus a completed lower is that you can complete the work yourself. Once you have done that, there is no need for the gun to have a serial number or be registered as long as it remains in your possession for personal use.