Who needs Vista:
Defend against attackers with no effort at all. One of the most common security breaches occurs when a hacker’s code calls a known memory address to have a system function execute malicious code. Leopard frustrates this plan by relocating system libraries to one of several thousand possible randomly assigned addresses.
And for those people who have trouble with the default Windows 2003 security policy requiring signed SMB packets:
Windows SMB Packet Signing
Enjoy improved compatibility and security with Windows-based servers.
Lots more fun at: http://www.apple.com/macosx/features/300.html