iMovieHD project as a high quality MP4 or h.264 file, then playback - by computer (or AppleTV, I think) will be HD. Check the "FAQ" at vimeo.com on the 720p HD they allow you to upload.
When you do this and copy to DVD, there are no regular DVD player that can read those files. These are computer readable. My HDTV has a VGA connection to which I can connect my Mac. It looks great. (Note: you need to connect audio, too.)
You CAN get a BluRay burner for your Mac...
It's been available for a long time... There are others - I like LaCie.
DVD Studio Pro (bundled with FinalCut) may be needed for rendering to BluRay (AVCHD) format readable in BluRay DVD a player connected to a HDTV.
I just got the Cannon HF10 High Def Camcorder and I have a few questions?
When I record in HD and then edit in iMovie and Burn in iDvd it converts it to a lower definition... okay, but would the quality of the video be better if I just recoded in that lower definition instead of letting iDvd do the down-grading?
Also, is there any hope in sight to be able to burn to Blu-Ray on the Mac anytime soon?
Thanks for any advice!