Air Canada and US Airways have an odd relationship. Consider:

You can codeshare on United and Continental but not US Airways
You can buy tickets on United / Continental flights on aircanada.com through codeshares but must call reservations to get US Airways flights
As a top tier member you get mile bonuses with United and Continental but not US Airways
US Airways gate agents despise Air Canada based on the couple of times I've gone to CLT through YYZ
US Airways preferred members get bonus mileage on United and Continental but not Air Canada
US Airways codeshares with United and Continental but not Air Canada

What's with the bad blood? Air Canada should codeshare with US Airways. It's a hassle having to call reservations to ticket it and I can't see the pricing!