The nexus s will only be compatible on Mobilicity or Wind Mobile, which both operate on the AWS band same as the T-Mobile version. Regardless though, you will still be able to use voice text and data on Rogers, Telus, Bell, Fido, Koodo, and Virgin. Rogers and Fido support 2g data for the nexus s while telus and bell have no 2g network since they recently changed to HSDPA (sim cards) so they don't have the network that Rogers/Fido had built years ago. I'm using my unlocked European HTC Desire on Rogers and it is definately usable on 2G Edge. Speeds aren't impressive, 0.20 MB/S to be exact most of the times but at the same time, I've tried on my friend's phone with 3g support for rogers and they could only reach speeds of 0.20-0.40 MB/S which is fairly disgusting. Webbrowsing, facebook, are all fine to use. One note is that youtube/video will NOT load as they take atleast 10min to buffer and usually will say unable to play video.
That being said, the nexus s will most likely get an AT&T version because of the exclusivity to T-MOBILE at the moment. If they don't, get the T-MOBILE Nexus S if you really want. If its a great phone to buy (not that I can recommend due to the horrifying construction of the body and I hate Samsung in general) then purchase it by all means.
Hope this helped