An official set of release notes accompanying the update says fixes are included for:
AFP, SMB/CIFS, NFS and FTP network file services?
AirPort and Bluetooth wireless access?
Core Graphics, Core Audio, Core Image, and RAW camera support?
disc recording when creating and burning media?
.Mac sync services?
Spotlight indexing and searching ?
Dashboard widgets: Dictionary, Flight Tracker, Stickies, and Unit Converter?
Address Book, AppleScript, Automator, Dictionary, Font Book, iCal, iSync, Mail, and Safari applications?
Disk Utility, Keychain Access, Migration Assistant, and Software Update?
compatibility with third party applications and devices?
previous standalone security updates
