I don't think Win8 will overtake Android or iOS in the tablet or phone arena. Windows is too bloated and inefficient. No way I see Win8 working well in a tablet form. MS has been trying for years with their phones and they've gotten no traction. However, let's have an imaginary world where Win8 works similarly well as iOS and Android on a tablet. If that were to happen, they would still have a hard time catching up to the App Store or Android Market. No apps equals no success as evident by WebOS. People seemed to have liked the OS, but no apps to support it.
I think that the Kindle Fire did well because it was backed by a very known name, Amazon, was in a form factor that had a lot of appeal, was tied to a lot of media (music/movies/shows/books), had a great portal to Amazon's shopping experience and was priced very well. I think the price point was a big factor in people's decision to go with the Fire instead of the iPad. Even if Apple comes out with a smaller form factor, I'm not sure if it would tip over the decision against a Kindle Fire if any smaller iPad is still $100+ more expensive.
What's going to dominate the tablet arena in 2012? I really have no idea, but it's going to be very interesting to see. I can't wait to figure out what my first tablet will be. I'm hoping something will come out and get me to buy it in the next month.