Apple recently posted an article on 64-bit support in Tiger. It looks like only a few libraries are available in 64-bit, so you can't make GUI apps that use a 64-bit address space. However, you can make a 32-bit GUI app that uses your favorite form of interprocess communication to talk to a UI-less 64-bit app. This seems like a reasonable short term solution, although it will be a bit of a pain for application developers that need a larger address space for existing applications. It may require some major refactoring.