from the US web site:
In the event your original US Airways flight is canceled, and you are rescheduled on a new US Airways, US Airways Express or US Airways Shuttle flight, you will receive mileage credit for the new itinerary. If you are rescheduled on another carrier that is not a partner in our program, you will receive mileage credit for your originally scheduled US Airways flight(s). If you elect not to travel, you will not receive mileage credit.
Just log in to your account and enter the ticket number, hopefully it will work. The online request form for mileage credit requests is more sophisticated than it used to be....you might be able to do it on there now but if not, you might have to fax the other airline's boarding passes to the DMSC for consideration with a short explanation (the US ticket number might help).
Believe it or not, one time my DCA-IND was cancelled and I was put on NW DCA-DTW-IND, and my next DM statement actually showed the NW flights and associated mileage! Because I got a couple more miles this way, I didn't complain. My NW boarding passes even had my US DM number on them.....I have no idea what was up with that! (I would have liked to have gotten NW miles as well as the US miles.) As noted above, the current protocol is different than what happened to me 7 years ago though....