Apple has released iMovie ’11 9.0.8 with a typically cryptic note about the update: “Addresses an issue related to third-party QuickTime components that could prevent iMovie from opening.” However, according to this support page, it appears that one of the third-party culprits is the 3ivx component (used to enhance compatibility with such camcorders as the Flip Ultra and Flip Mino). If you currently have 3ivx installed, version 9.0.8 of iMovie displays a warning dialog that explains how to remove the component. ($14.99 new from the Mac App Store, free update through Software Update or the Mac App Store, 1.08 GB)
Open a Finder Window with the App Switcher
If you don't have any Finder windows open and want to switch to the Finder and open a window in one fell swoop, bring up the app switcher with Command-Tab and tab over to the Finder icon. While still holding down Command, press the Option key, and then release the Command key. You're switched to the Finder, with a new default window open for you.