Do you require TN status? It is relatively easy to get, however I understand you can only receive it once. Getting legal status after that year is considerably harder, so be careful when you use it!

Offtopic: Why do you need to leave Canada to start it?

There are a few good entrepreneurial hubs in Canada, like Ottawa for telecom and Waterloo for Engineering/Software.

It's actually possible to renew TN status indefinitely, as long as one maintains temporary intent. I've been living in Boston a few years, working on a TN for a slightly larger company, and haven't had problems at the border.

However, the situation potentially gets a little trickier when a start-up is involved, as one is not supposed to be self-supporting - you need to have a paying job. However, I believe that in principle, there isn't any reason that one can't hold an equity stake in a company that employs you.

I am already in Boston and working with a co-founder here, which is the biggest reason I need to be in the US. Moreover, the access to smart capital and to our target market is significantly greater here (Boston, SF, and NYC). Personally, I have found the opportunities for inspiration, advice, and serendipity are much richer in Cambridge than they were in Toronto. That isn't to say it can't be done in Canada, but at this stage, my bets are best in the US.