Apple released iMovie 10.0.1 on Wednesday. The update adds support for "some older video cards," and it addresses unspecified problems some users have when working with projects created in earlier iMovie versions.
Apple's patch notes:
- Allows iMovie to be installed on computers with some older video cards
- Addresses reliability issues when updating projects and events from previous versions of iMovie
The update is available through the Mac App Store. It's a huge download, at 1.94GB, so make sure you're not downloading on a slow network or over a cellular connection.
As of this writing, Apple hasn't yet added it to it Downloads site.