Aluminum wire is still approved by the N.E.C. But this on the assumption that everything will be done perfect. Having said that i would like to give a little advice on using aluminum wire when you are building a new home. For the sake of saving a few dollars, it is not worth the safety hazard you face when using this wire. This is not only my opinion but the opinion of many others in the trade. Many times i have been called to repair corroded connections using this wire. Most of the time the aluminum wire had to be replaced with copper. One other piece of advice, before you consider buying a home with aluminum wire you might want to check with the insurance company. Some of them may refuse to insure a house with aluminum wire.
 There are a lot of homes that have been wired with aluminum wiring and it would not be financially possible to rewire the entire house or service. There are a few things you can do to make sure that the wiring is not becoming a problem. You should be on the lookout for devices or lighting going on and off. Breakers or outlets overheating. Have the main panel checked for corrosion or loose connections. When adding copper devices or wire to aluminum, make sure this work is done by someone who knows the proper procedure for this type of wiring. A little preventative maintenance can go a long way in preventing future safety hazards.

The article below was sent in by a rep. for an aluminum company.
Bob, Thanks for the information you've made available on your site. If possible, you may want to update the information regarding aluminum building wire. Nearly 95% of residential homes built today are using aluminum cable for Service Entrance. The aluminum building wire in use today (the only one approved by the NEC) is the 8000 Series aluminum alloy. Even though this cable is approved for sizes 12 AWG and larger, no manufacturer produces sizes smaller than #8 (for SEU cable)and #6 AWG for single conductor.
  I agree, that if someone has aluminum cable in a house older than,say, the mid eighties, chances are it is NOT 8000 series. The previous product was actually the same aluminum utility cable (EC1350) that is still used on power lines today. However, it was a mis-application to bring this product into a house for switches and receptacles. The Aluminum Association, along with cable manufacturers, and CONNECTOR manufacturers set out to rectify this problem, resulting in the 8000 series alloy in use today. Here in Florida, projects ranging from single family homes to Raymond James Stadium, are incoporating 8000 Series for electrical feeders. Just thought you should know.

Here is another opinion from an electrician.
I think your stand on aluminum wire is wrong or stated wrong. I would never use copper in a direct burial service wire. In a case where the wire insulation is damaged underground, both will fail. But copper will "spin" the meter for much longer, causing brown-outs and possible equipment damage over a longer time.