Hulu has apparently been doing this for a long time, but I haven't noticed.
Like I've said in other posts here in the past, determining location based on your IP address is a really hard problem to solve because IP addresses weren't ever supposed to have any association with a physical location.
Ultimately, the companies like Unblock-Us have the home field advantage.
If Netflix is okay with "false positives" (i.e. blocking some of their customers in the USA, collateral damage), then they will probably be able to effectively block Canadian users using USA Netflix too. Or, at least they'll make it really annoying for the proxy companies.
We'll see. Maybe Netflix has a magic bullet, but I somehow doubt it.