"For most of my lifetime, it's been a given that the top Canadian talent in any field would eventually move to the States, or spend significant time there."

I think that is true in some fields, like the performing arts and untrue in others like engineering esp. telecommunications where top talent is homegrown thanks to the likes of Nortel and now RIM. These people do not want to move and there is a critical mass that hasn't gone away even after the bubble burst.