If you’re having troubles with iMovie HD (and you’ve already repaired permissions on your Mac), check to see if you have any third-party iMovie plug-ins installed. GeeThree has run into a few conflicts, and posted a page detailing current problems and workarounds.
Unfortunately, this is one byproduct of Apple’s product secrecy; as far as I know, no third-party developers received early copies of iLife ’05 to test their products against, so the first several weeks are usually spent catching up and sleepless.
