http://www.apple.com/macosx/leopard/64bit.html
"Leopard delivers 64-bit power in one, universal OS. Now Cocoa and Carbon application frameworks, as well as graphics, scripting, and the rest of the system are all 64-bit. Leopard delivers 64-bit power to both Intel-
and PowerPC-based Macs, so you don’t have to install separate applications for different machines. There’s only one version of Mac OS X, so you don’t need to maintain separate operating systems for different uses."
Before they updated the site, it said...
"Now that the entire operating system is 64-bit, you can take full advantage of the Xeon chip in Mac Pro and Xserve. You get more processing power at up to 3.0GHz, without limiting your programs to command-line applications, servers, and computation engines. From
G3 to Xeon, from MacBook to Xserve, there is just one Leopard."
The earliest processor it will run on will be the G3 processor. The latest will be.. well, obviously the Intel ones.
See?