How hard is it for a graduate from Canada to find an employment in the US?
Not exactly an advice, but just based on what I know of my friends working in US.

I am not familiar with international visa issues, but I have at least 2 Canadian friends (as far as I know they don't have US citizenship) who graduated from Canadian schools and now work for Google and Twitter (actually one of them has BSc). So I agree with scaaahu's comment to your question, that it is much more important to have the skills that your potential employer wants.