Telus and Bell use CDMA technology and will only activate their own phones. Even though technolgy is same, they use different software/firmware, so will only activate their own phones. Hence very little market for used CDMA phones. I believe Verizon and Sprint in USA are CDMA also. CDMA is not that popular outside North America.
Rogers and Fido use GSM. Any "unlocked" GSM phone from a US carrier can be used, by simply inserting the SIM card. If the phone is not unlocked - it will not work. There are numerous places in Toronto that will unlock GSM phones. Cingular and T-Mobile are GSM carriers in USA. Europe is mostly GSM, but operate on different frequencies than north America. A quad band GSM phone will work world wide. Some GSM phones sold in North America are only dual band or tri band.