Apple just released their latest updates for Leopard 10.5.3. Also today they released their latest, and probably last Beta version, of their developer tools XCode. The latest version the tools requires 10.5.3. The final version of the developer tools is expected to ship on the first day of the WWDC conference on June 9th.
Download the latest Mac OS X update either from Software Updates or from here http://www.apple.com/downloads/macosx/apple/macosx_updates/macosx1053update.html.
Download the latest Beta version of the developer tools here http://developer.apple.com/iphone/program/.