The new mobile carrier in Canada. And the only one in Canada that the Nexus One's 3G is compatible with (AWS - 1700mhz).
The Nexus One on Rogers, etc... will only allow you to use EDGE for data. You can still use your current SIM, but data transfer will be stuck on 2G slow shit. And for a phone built to be connected to the net 24/7, buying just doesn't make sense if you're not on WIND.