One of the reasons the big SF/Valley companies are opening Canadian offices is that even Canada's most expensive cities (Toronto, Vancouver) are a relative bargain. Engineering salaries are lower, rents are cheaper, and government money is plentiful (SRED, etc).
I think it will do two things to Vancouver's startup ecosystem. In the short term it will be painful as you lose talent to the better funded new competition. In the long term it is a win as the big companies validate the Vancouver 'brand', increasing exposure and attracting new investment.
