The 10.6.4 update is recommended for all servers currently running Snow Leopard Server version 10.6.3. It includes Wiki Server Update 1.0, Safari 5 and general operating system fixes that enhance the stability, compatibility and security of your server, as well as specific fixes for:

CalDAV querying of email addresses with Active Directory and Open Directory servers.
Stability and reliability accessing Wiki Server pages.
View movies on Wiki Server with Safari on iPad.
Accessing publicly accessible Wikis when logged in to the Wiki Server.
Listing blogs on Wiki Server.
Vacation mail notices that span across multiple days.
Updates from Software Update Server viewable by clients.
Stability and reliability to Software Update Server synchronization.
Downloading of software updates by Software Update Server.
Creating NetInstall images which include iLife updater packages.
Address issues copying, renaming, or deleting files on SMB file servers.
Improve reliability of VPN connections.
Resolves compatibility and performance-related graphics issues.
Improves compatibility with large-format SDXC memory cards.

System Requirements
Mac OS X Server 10.6.3