Apple has released GarageBand '09 5.1 [1] to fix a security-related bug in the popular audio editing software, enhance a few features, and address other issues.
Most notably, GarageBand '09 5.1 fixes an issue that could lead to your Internet activity being tracked by third parties without your knowledge. Previously, when opened, GarageBand '09 would change Safari's preferences to accept cookies always. The default cookie setting, which offers additional privacy, is that cookies are accepted only from sites you visit - see Wikipedia's discussion of third-party cookies [2] for more information. While GarageBand '09 5.1 no longer makes this mistake, if you've run a previous version of GarageBand '09, you should confirm that your cookie settings are set as you wish in the Security view of Safari's Preferences window.
In addition to fixing this bug, the GarageBand '09 5.1 update "addresses general compatibility issues, improves overall stability, and fixes a number of other minor issues." Those minor changes include the added capability to add GarageBand track effects and Audio Units to a guitar track, enhanced Apogee audio interface support, better access to audio monitoring settings, and speedier switching to full screen in Magic GarageBand.
Apple recommends the 139.3 MB update for all GarageBand '09 users, and it is available via Software Update or from the Apple Support Downloads site [3].
[1]: http://www.apple.com/ilife/garageband/
[2]: http://en.wikipedia.org/wiki/HTTP_cookie#Third-party_cookies
[3]: http://support.apple.com/downloads/GarageBand_5_1