What you can do is purchase an phone on a 2-year contract (like a Palm Pre or an iPhone 4 when it comes out), sell it on eBay for around $500-$600, and use that money to buy an unlocked phone. I suggested this to one guy on this board, he found a Palm Pre for one penny (doubt that is going to be available anytime soon), sold it on eBay and now he is looking to buy his phone.
The one major advantage of GSM is that you do not need to purchase a phone which is in their system, just one that works with the system. The Nexus One is available with AT&T 3G bands (which btw are 850 MHz/1900 MHz. 1900 is the stronger one as of right now but I believe they intend on building their LTE network on the 850). Most Canadian carriers use the same 3G bands as AT&T (and they have alot of great phones in Canada such as the Motorola Milestone, Acer Liquid E, HTC Desire/Triumph is coming, HTC Legend and many more).
I bought an Acer Liquid A1 for $360 on the internet. It works with the AT&T 1900MHz 3G band (and so far, my 3G has been strong in most places throughout the greater Houston area). The Liquid E (which released in Canada today) is pretty much the same phone only with 2.1 (I had to install it manually, then root it. The Liquid actually has a strong development community) and 512 megs of ram as opposed to 256.
Honestly tho, if Sprint is strong in your area, stay on Sprint. With WiFi, they will be getting the best subsidized phones. AT&T discontinued unlimited data today and Verizon is expected to follow suit. Then again, I use alot of data and not everybody does lol.
Black AT&T Motorola Atrix 4G
ROM: CM10 by Epinter
White Acer Liquid A1
ROM: LiquidNext 1.9.2 @ 998MHz by Suxsem, ThePasto