I think many editing programs now days are HD. iMovies last version was called iMovie 6 HD. I think the latest iMovie 9 is also HD. If you want to step out of editing geared for simplistic home videos, try Final Cut Express, which is available for both PC and Mac. The Express version is a scaled down version of Final Cut Pro. FCP will take you to a whole new level of editing. The other option may be Sony Vegas Movie Studio HD.
I used to shoot video on MiniDV tapes and edit them in iMovie on a Mac that I bought in 1998. It was great fun but I think I enjoyed the editing most of all. Typically, the finished movie was about 30 minutes long. Now, I'd like to move to HD and am curious about how the editing process works with the new capture devices and which editing programs people might recommend. My computer now is a PC running Vista with plenty of memory and tons of storage.