Overshadowed by the release of iTunes 4.5 is a new version of iMovie (4.0.1), available via Web download. According to Apple, the update provides "improved performance and reliability." Specifically:
This update provides increased performance when working with large projects, such as editing video clips and audio volume. iMovie 4.0.1 provides increased reliability when the application opens, when titles are added to the timeline, and with 3rd party plug-ins that are removed or no longer available to an iMovie project (for example, when you open a project on a different computer). Other improvements include fixes for pixelated video, corrupted Ken Burns clips, and loss of audio, which could occur after editing video or audio and then emptying the trash.
Note that iMovie must be installed in its original location in order for the update to be successful.