Yet another 5-door liftback seeker. Sophie, I've been looking at the same thing myself and my conclusion is that it's certainly possible, but you have to weigh the extra cost to import one against the real possibility of Toyota introducing the 5-door liftback in the states next year (I'm keeping my fingers crossed).
As far as the logistics, take a look at these NHTSA importation guidelines specifically for Canada:
http://www.nhtsa.dot.gov/cars/rules/...nada07032.html
From looking around on the web, it sounds like most dealers will refuse to sell a new car to a US citizen, so you may need to have a friend buy it and sell it to you. Also the speedometer should be ok as long as it has mph markings.
Pricewise, if I remember correctly, there was about a 3-4k difference in price between the 3-door US Yaris and the 5-door Canadian Yaris (go to yaris.ca to price one out).
That's just a bit of what I've found online. I would love to hear more from anyone who has succesfully imported a vehicle from Canada.