I'm switching from OS X 10.6 to 10.8 and intend to keep using iMovie HD 6.0.3 and iDVD with my DV projects.
iLife '06 installer can't run in OS X 10.8 so I must either:
a) 1st install iLife '06 while booted in OS X 10.6. Then copy /Applications/iMovie HD.app and /Library/Application Support/iMovie to the same places in OS X 10.8. Or:
b) Use Pacifist ($20) to open the iMovie HD 6 install packages from the iLife '06 installer DVD (iMovie.pkg and iMovieThemes_Tiger.pkg) and let Pacifist install them to the right places.
iLife '06's iDVD 6.0 (or 6.0.4) does not work in OS X 10.8 so I must install iDVD 7.0.4 (and iPhoto 8.1) from the "Applications Install Disc" DVD that came bundled with my Mac mini. Remember to choose "iDVD extra content" which installs iDVD 1-5 themes in addition to the default iDVD 6-7 themes. No need for Pacifist with this install.
iMovie HD 6.0.2 and 6.0.3 as well as iDVD 7.1.2 and iPhoto 8.1.2 updaters also work without Pacifist. AFAIK iMovie HD 6.0.4 is identical to v6.0.3.
So far all those old apps seem to work OK in OS X 10.8.
Oh, and BTW I had to grab iSync from OS X 10.6 for use with my ancient Nokia E70 "smartphone", too.