From my calls in to various US dealers, I discovered that they will sell used to Canadians, and if you look hard enough, there are lots of low mileage gems to be had.
That being said, after factoring in duty (6.1 odd %), taxes and shipping, the cost savings weren't that significant. Ultimately, I wanted a new car, and with some negotiating was able to get a reasonable discount on a new-build right here in Calgary
Make sure that you're comparing apples to apples when looking at US vs. CDN M3s. If resale is a factor, bear in mind that US vehicles will command a little less up here if you ever decide to sell.