If you would like to stay with AT&T (a North American GSM operator) and Telus (a CDMA operator), you will have to use a different phone for each of them due to the fact that they use incompatible technologies (GSM and CDMA).
CDMA operators do sell world phones that have both GSM and CDMA modules. But they are designed to be used on GSM networks outside of North America. In North America, GSM operators use a different frequency than other parts of the world. So these phones will not work with them.
I want a Blackberry. Very badly. My AT&T contract is about to expire and I'd love to get the Blackberry Tour. However, I spend half my time in Canada and half in the US. I stupidly signed up for Telus, which is the equivalent of Sprint, rather than Rogers, which is AT&T's partner. If I get a phone from AT&T (which uses GSM), will it be locked, preventing me from using it with Telus (which uses CDMA)? I'd prefer not to buy an unlocked phone from Ebay because it will be so much more expensive. Any feedback would be greatly appreciated! Thanks so much!