Adding to what the others have said, Mississauga is the western suburb of Toronto. Solid network coverage.
Rogers, Bell, and Telus are the Big Three companies.
Bell was the Original Phone Company in Ontario before competition; Telus was the Original Phone Company in Alberta, but now they compete with each other and with Rogers.
Rogers was the Original Competition. Rogers built its own GSM/UMTS network. Bell and Telus have competing CDMA networks in different areas, but they went together and built a nationwide UMTS network to directly compete with Rogers (and also offer the iPhone). Note that Bell and Telus don't have GSM to fall back on if there's no UMTS available; Rogers does.
Fido, Koodo, and Solo are the discount brands of Rogers, Telus, and Bell respectively. They share their parents' networks.
Virgin is a Mobile Virtual Network Operator (MVNO) that uses the Bell/Telus UMTS network, and it too will be offering the iPhone 4. There are several other MVNOs out there, like Canadian Tire, President's Choice, Cityphone, and others, that ride on the networks of the Big Three.
Fido was originally a separate company with its own network, the first GSM network in Canada. It was bought by Rogers and integrated into the Rogers network; later, the branding was changed from 'stylish cosmopolitan professional' to 'black, yellow, and inexpensive'.
Wind, Mobilicity and Public Mobile are all new companies. They are building their own networks and their coverage is confined to the biggest cities, but I believe they can roam on Bell or Rogers.
Public Mobile is CDMA, so you can ignore them. Wind and Mobilicity are on the AWS-1 band (UMTS band IV), using 1700 MHz up and 2100 MHz down. The iPhone will not work on their networks. (I was really hoping that the iPhone 4 would have 1700, like the Nokia N8 does, but no such luck.)
So that's what the landscape is like. If you're just going to be in Mississauga and downtown, every company will have coverage. Choose rather by feature and plan.
Canadian phone companes aren't used to this free competition yet, and selling SIMs for unlocked phones is a little new to them. You may have to be persisitent in order to get a SIM for an unlocked phone. SIMs can be found on the websites of Wind, Rogers and Virgin, and are at least mentioned on Mobilicity's website.
Useful link: Howard Forums, a mobile-specific board.
http://www.howardforums.com/
Komiksulo, Fido customer since '97, when they launched in Toronto.
