This will get an international release eventually, Amazon just won't say when yet. I suspect that before they launch in Canada they'll want to make more of their content services available here. I doubt they make much (if any) money from the hardware; they need to be able to sell their content.
I'm sure this will put a dent in iPad and Android tablet sales, but I suspect the biggest impact will be on the cheap, low-end Android tablets. This device may have limited abilities compared to an iPad or Xoom, but it is low-cost, powerful, and supported by a huge company who will be very concerned about perfecting the experience. If you just want a low-cost tablet to browse the web and use a few basic apps (email, facebook, etc) then this makes way more sense than an iPad or high-priced Android tablet. You can buy two of these for the cost of an iPad, and still save $100. But more than that, for the average user it can probably do everything that a low-cost Viewsonic or Archos Android tablet can do, but much better. And Amazon can sell these at a price point that nobody, not even Apple, can match because they'll make their money on content.