Well, it would make sense. Amazon's Kindle app on WP is much better than its Android version, so we know that they have the technical know-how to move into that direction and they would steer clear of all the legal issues that B&N is facing now.
Kindle Fire is an Android based tablet and is just as targetable as the NOOK devices. So either Amazon is already not steering clear, or they've paid the patent fees and are safe. So this seems like faulty reasoning to me. Given Amazon is already in the Android game, and would gain revenue throug it's App store, I'd find it unlikely that they'd have an Amazon branded WP7 device.