It's not restricted to US only, but all of the NANPA. North American Numbering Plan Administration covers everywhere that uses the country code +1. This includes all of Canada, many islands and territories, and a few other exotic locations.
I just tried it for some numbers around Montreal. Apart from the delayed results issue, it works.
I just tried my home number and the name it came up with was 'Nova Scotia'. My landline provider is based in Halifax, but I'm in PEI. I'm kind of used to that, though :)