iMovie version 9.0.8 fixes a minor issue regarding third-party software components that could cause Quicktime to stall on launch.
From the release notes:
About iMovie 9.0.8
What's New in Version 9.0.8
Addresses an issue related to third-party QuickTime components that could prevent iMovie from opening.
The update weighs in at 1.08GB and can be downloaded through the Mac App Store or directly from Apple's support page.