from macosrumors.com:
Apple's emphasis in the 10.5 era will be on resurrecting 'Yellow Box for Windows,' a set of Cocoa (and potentially also Carbon) API's for Windows that would allow Universal Binary applications to run on Windows with a mere 150MB software package installation. And best of all, there is no extra work to be done on the developer's part to get fully native, rock-solid stable performance from their Xcode-developed Universal applications on Windows