I just installed iMovie '11 and I can still use iMovie HD just fine...the iMovie 11 installer put it in a folder called "iMovie (previous version)", just go to that folder and you can open it right up.
I had hoped that I would not need iMovie 06 anymore after buying iMovie 11 but unfortunately there are still serious quality issues if you are editing anything other than HD video.
For those who are interested, this is how I was able to maintain full quality for 480p video by editing in iMovie HD 06 (note that the easier way is just to purchase Final Cut Express…if you additionally need to be able to edit in 24fps natively, you need Final Cut Pro):
1. Convert the video to DV at 29.97 frames per second, which is iMovie HD's native format.
2. Import the video into iMovie HD
3. You'll notice that iMovie re-encodes the video in DV format, resulting in a slight quality loss (even though it's converting from DV to DV, anytime you re-encode you lose a little quality). If you want to prevent this, you can control-click on the iMovie HD project file in the Finder and select "Show Package Contents." From there you can go to the media file and replace the DV file with the original - as long as it has the exact same name, ending with the .dv extension, iMovie HD won't know the difference.
4. Edit as usual
5. For exporting, it's best if you have QuickTime Pro, otherwise you lose a little quality on the export. But if you have QuickTime Pro, then in the export window you can click on the QuickTime tab and choose "Expert Settings" from the drop-down. Click Share and the QuickTime export window will come up, then click on Options and export using Apple Intermediate Codec. Apple Intermediate Codec is great because you can export with full quality, and then do your encoding for the web or other formats with a more suitable tool like Adobe Media Encoder or Compressor.
P.S. iMovie HD doesn't let you edit natively at 24 fps like iMovie 11 does, but if all you need are simple fade-ins/fade-outs then you can telecine the video using JES Deinterlacer before importing to iMovie HD and then inverse telecine it after exporting, meaning that you'll still get a 24fps file in the end without losing any quality except for some minor interlacing in the fade-in/fade-out parts.
"Was iMovie HD (also known as iMovie HD 6.x) the only version of iMovie on your machine prior to installing iLife '11? If so, it wasn't in the 'Old iMovie' folder that the iLife '08 install would have created when it installed iMovie 7.x and the iLife '11 installer might not have been smart enough to move your iMovie HD into such a folder to save it."
