The 800 coming to Microsoft Store's for ATT will be optimized for their network. So I can see Nokia optimizing the 800 for Telus as well since they will be the exclusive carrier in Canada. Once its released we'll know for sure. But I can see that as accurate.
And yes I'm certain if you buy from Telus it will be locked to them. That's been the case with pretty much all their phones and I don't see this being any different.
I'm really considering this. I was going to wait and see who gets the Lumia 900, but Telus has an awesome promotional plan right now with 6 GB data at $70/month. Wow.