Apple has published a series of free PDF documents that cover the security features of Mac OS X:
Mac OS X Security Technical Brief (Leopard version): http://images.apple.com/macosx/pdf/MacOSX_Leopard_Security_TB.pdf
Mac OS X Security Configuration (Leopard version): http://images.apple.com/server/macosx/docs/Leopard_Security_Config_20080530.pdf
Mac OS X Server Security Configuration Guide (Leopard version): http://images.apple.com/server/macosx/docs/Leopard_Server_Security_Config_v10.5.pdf
Smart Card Setup Guide: http://images.apple.com/server/macosx/docs/Smart_Card_Setup_Guide.pdf
With the release of Snow Leopard, Apple will likely add PDFs to address security features of the new platform.