Apple have released the 10.5.6 update. Whats covered:

• Address Book - syncing improved
• AirPort connections
• iChat
• Graphics - including fixes for possible graphics distortion issues with certain ATI graphics cards. I'm hoping this means fixes to horizontal black lines I keep getting
• Mail
• MobileMe
• Networking
• Printing
• Parental Controls
• Time Machine
• and a few other general fixes

You can find a more detailed list at http://support.apple.com/kb/HT3194