Very true. The very basis of it works like this:
Right now, your average DVD's run at about 720x480 resolution (roughly, it'll change depending if you're using PAL or NTSC, people in America will be using NTSC). This is because, this is the highest resolution a normal TV can handle (no matter the size, from 14 inch to 19 and above, if the TV isn't HD ready it will be this resolution and on very large TV's, it won't look nearly as good).
HD TV's, though, require a lot more information for movies and so on to be HD, they run between 1280×720 and 1920×1080 (although higher is expected in about 2015). They also have a higher framerate (i.e, how many times a second the screen refreshes) to acommodate a larger picture. With all this considered, on such a TV, you'll see a large drop in quality when viewing a normal TV, when compared to Blu-Ray or just HD TV.
Again, as Thrik says, if this kind of thing really doesn't bother you, then wait it out (eventually everyone will be using HD, but that won't be for many years), but for those of us whom enjoy a good experience out of our home cinemas, it has come to be fairly important to us.