Perhaps silently acknowledging that iMovie ’08 is a completely different approach to video editing, Apple has made iMovie HD 6 (the previous version) available as a free download. The release note states that it’s available “for those who have purchased and installed iLife ’08.” I’m still downloading the 154 MB file (fairly slow DSL at home), so we’ll see if there’s something in the installer that checks for the presence of iMovie ’08.
Updated: Yes, the installer checks to see if iLife ’08 is installed before continuing. This means that anyone buying a brand new Mac that comes with iLife ’08 (and who therefore wouldn’t have a previous copy of iMovie HD 6) could download this installer and get iMovie HD as well. But if you’re still using an older version of iMovie, you can’t upgrade to iMovie HD 6 for free.