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...
http://www.lacie.com/us/products/product.htm?pid=11054
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.