Its technically feasable. The Nvidia chip in the AppleTV should be able to do it, but I'm betting that Apple stuck with 1080i because some programming looks better on it, plus they can guarantee perfect frame rates.
ummm, what are you talking about? For the 1080i output on the machine to work, everything must be scaled to 1920x1080, also known as 1080P, after that it is interlaced. There is no technical reason why 1080P was not included.