On the heels of the recent iDVD update, Apple has released a new version of iMovie, which addresses a problem with audio playback being out of sync. In addition, the update addresses a number of undisclosed stability issues. The update is available for all users of iMovie '11 (version 9.0.0 or greater) and requires OS X 10.6.3 or later to install.
The update should be available through Software Update, but can also be downloaded from Apple's iMovie 9.0.2 update page (27.52MB).
In addition to the iMovie update, Apple has released the latest set of Brother printer drivers for OS X 10.6, which includes support for the latest models of printers and scanners from Brother. The update is 123.84MB, and updates to installed drivers should be available through Software Update, or you can download and apply the full package from Apple's Brother printer driver download page.
In the past, people updating printer drivers have experienced problems that have required the print system to be reset, so unless you are currently missing features or need specific support supplied by the new drivers, then you might consider avoiding the driver update.
As with all updates, be sure you fully back up your system before applying them. Use Time Machine or a cloning utility to make a full system backup so you can either restore iMovie projects or the whole system if needed in the event of an unforeseen problem.