To be clear.... to do 64 bit integer arithmetic (internally) an application does NOT need to request 64 bit virtual addressing.shamino said:64-bit application code is not something most people need.
It is needed for apps that require more than 4G of memory (64-bit memory pointers), and it is very useful for apps that do a lot of 64-bit integer arithmetic (scientific applications, simulations, maybe some games.)
Mac OS X as of 10.2.8 (and matching xcode) has supported the use of 64 bit integer / general arithmetic when running on a G5. As of 10.4 Apple added support for applications to request a 64 bit virtual address space, this later aspect was broken by mistakenly not including the 64 bit version of libSystem in the first security update.
(possibly because of the update system not being configured to correctly deliver the package containing 64 bit libSystem to folks that have G5s)