Quick question: Buying movies - HD?

No HD movies are only available for rental, it's more than likely a restriction placed on  by the movie studios.

HD movies are not currently available for purchase in the iTunes store. If you purchase a movie it will be in Apple's famed "near DVD quality." I have purchased a number of movies from the iTunes store and, while the quality is certainly acceptable, it's obviously not HD. It's anyone's guess as to why they don't sell HD movies, but I expect they will offer them at some point in the future.

The inability to purchase HD movies on Apple TV will surely send the whole Apple TV platform into obsolesence quicker than you can say "Blu-Ray"

The lack of HD movies for purchase is a restriction that I am sure is placed by the movie studios, at least for now. As far as I know, none of the other online movie rental services, such as Netflix or Roku, offer HD movies for sale, either. With HD TV shows now available for purchase in the iTunes store, I think it's only a matter of time before movies follow suit.

As for becoming obsolete, various reports have it that Apple TV sales have actually been on the increase since the movie rental service started. I'm generally inclined to rent movies rather than buy them because I rarely watch the same movie more than once.