Steve, while it did wander a bit, the topic didn't get too far off track.
The 'simple' answer is: Yes, the NEC allows aluminum wire to be used for feeders and branch circuits. This 'simple" answer, though, is misleading.
It is misleading in that aluminum wire is not made in sizes small enough for the most common of branch circuits, the 15 and 20 amp 'convenience' receptacle and lighting circuits.
It is also misleading in that the great majority of breakers, lugs, and devices are not approved for use with aluminum wire. This puts you in the position of having run wire that you can't connect to anything.
There is also a bias in the replies - a bias founded upon a half century of experience with aluminum wires - that wants to discourage you from using aluminum wire at all, especially in branch circuits.
Also, if you have a specific application - say, a swimming pool - you may find specific NEC rules against the use of aluminum wires.
