Since this issue seems to come up a lot and many folks are not quite certain on what is right and what is wrong... The June, 2010 ATF Federal Firearms Licensee Newsletter translated the legislation and enabling regulation into plain English:

Non-Licensed Residents of Another State:
+ May dispose of firearms to a licensed firearms dealer.
+ May Not acquire firearms OR dispose of firearms to a non-licensed individual.
+ May Not engage in the business of dealing in firearms without being licensed.

If you are going to sell a firearm to a non-licensed out of state buyer, the sale must go through an FFL in the buyer's state.