Bruce - the cordless impacts have certainly become very viable in the past few years and more convenient to use than air. In fact, if you go to a NASCAR race, all of the behind the wall preparation, the teams are using cordless impacts because of their added convenience (no dragging air lines, etc.)
If you had to have something now, you can go to OSH as I believe they stock the 1/2" Dewalt impact. I recently did a lot of research as my old cordless tools were almost 6 years old and my batteries (NiCd) were all dead. As such, I wanted to re-invest in newer technology and select what I believe were the best tools out there.
I chose to go with the Milwaukee brand. I recently bought the Milwaukee combo set from Home Depot and then I added a 1/2" impact (450 lb-ft capability) by ordering through Amazon.
Here's the combo set I got:
http://www.amazon.com/Milwaukee-2691-24-Reciprocating-Circular-Worklight/dp/B001FA17Z6/ref=sr_1_8?ie=UTF8&s=hi&qid=1260546852&sr=8-8
(I bought it about 2 weeks ago from Home Depot for $399 but I don't see it there anymore.)
Finally, and more to your question, here's the Milwaukee impact:
http://www.milwaukeetool.com/ProductDetail.aspx?ProductId=2662-22&CategoryName=SC:+M18+Cordless+System