This would be a great start! I have always wanted to be able to rent movies and TV shows online and be able to watch on my 51" HDTV with little to no hassle of setting stuff up. If they introduce this, even just DVD quality to start, I'd buy an Apple TV the same hour of the announcement!
Knowing Apple, it's just a matter of time until they offered HD content. If this renting option happens, renting, and as an extension, buying movies online will be far more "mainstream" than it has been, simply because most people don't buy DVDs unless it's one heck of a fantastic movie. Most people first rent and then buy if they really love the movie.
Add in the convenience of one-click renting HD movies (hopefully!) from the comfort of your couch via Apple TV, and you can be certain that Blu-Ray and HD-DVD will be saying buh-bye sooner than later.