Don’t be shocked when the Apple television has a Retina Display as a key feature. Because your eye’s distance from the screen is part of the calculation, a television could fit the “retina” 300ppi minimum requirement easier than most other screens they make.
In fact, they already have more than enough pixels in the existing 24” iMac screen, so it’s just a matter of manufacturing it at 50”, rather than any major technological leaps.
If they do that, it could redefine the “HD” label. To put it in perspective, on a “retina” TV, a Blu-Ray movie (as HD as you can get) would still have the black outline around it like you get with standard-def on today’s HDTVs. (That’s with rough math, but you get the picture.)
That would leave the content as the weak link, but that’s where the existing Apple TV infrastructure could really help complete the package. It would take a while to prepare content at a whole new resolution, but who better than Apple to make that push?