Geekbench ran 64-bit benchmarks on Intel Mac's, so I am guessing that Tiger is 64-Bit on Intel Macs. http://www.geekpatrol.ca/2006/...vs-64-bit-performance/
Anyway, Leopard (OS X 10.5) is coming out in October, and Apple has said that it is 64-bit, "From the ground up." (Whatever that means...