That's because your Nexus One supports HSPA 850/1900. The Desire HD which the OP was referring to does not. Thus, only 2G GSM/EDGE would be available in Canada.

Unless a different version is released (like the original Desire), no Canadian carrier will support 3G on the HTC Desire HD. Only Rogers/Fido/Chatr can support it in 2G.

Updates would still be available, but they may not come OTA, and you would be at the mercy of a carrier in another country to release it. The Nexus One is different in that updates come directly from Google. At the moment, all other phones must pass manufacturer and then carrier certification.
@TorontoColin

Does your comment about OTA updates apply to the NexusOne acquired through Mobilicity or Videotron (the only Canadian carriers offering it at the moment, to my knowledge)? The Mobilicity one, I understand, is locked to the carrier's network and Mobi~ hasn't said anything about unlocking it [partly, I assume to keep its limited stock for its own customers].

I believe it does. As (to my knowledge) the carriers don't modify its software at all I can't see any reason why they'd need to approve an OS update. I suppose we'll see when the next one is released.

Mobilicity claims they're locked, but they also claimed that about the Sony Ericsson TM506, which wasn't locked. I have yet to hear from someone who actually bought it.

I can certainly see some outraged buyers if it does require carrier approval, particularly if carriers disable some popular features.