This thread has a huge amount of information:
http://www.flyertalk.com/forum/star-...nce-links.html

Kiwiflyer has already given some useful info.

Yes, you can fly CO and earn miles on US. Just give your US Air dividends miles to the CO gate agent or ticket agent before you fly. Soon, you will be able to input your USAir number when you are making the reservation on CO.
However, again, as Kiwiflyer pointed out, you can earn miles only on fares that allow one to earn miles.