Nice! I made the same switch about two months ago and the Nexus One will amaze you.
BTW I'm on Rogers in Canada and I was able to switch the SIM back into my 8900 on a trip to the U.S. recently to keep the data usage down (N1's a great phone but uses data like crazy compared to a BB).
Actually, Telus and Bell has GSM now but they're on the same frequency as Rogers and AT&T.
dan1431 is right. WIND uses AWS so you can use that for 3G in Toronto but I don't know what's available in Montreal.