Carlson is writing a book called "iMovie '09 & iDVD Visual QuickStart Guide". While updating the book he had to do a little searching into the software update.
He has found four changes worth noting:
Split Clip Keyboard Shortcut -- You can now split a clip by pressing Command-Shift-S instead of accessing the command from the Edit menu.
Interlacing Bug Fixed -- Apple fixed a bug where 1080i (interlaced) footage would appear combed when exported to QuickTime or iDVD.
Enter Coordinates for New Map Locations -- You can now plug in latitude and longitude coordinates into the Choose Location dialog when creating a map and iMovie sets the destination.
HD Export Options -- When sharing to YouTube or choosing Share > Export, a new HD option is available that creates a 720p (1280 by 720) movie with a bitrate of 10 Mbps.