Well, a guess a lot of G3 users will breath a sigh of relief:
http://www.apple.com/macosx/leopard/64bit.html
Bridge the Generation Gap
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.
http://www.apple.com/macosx/leopard/64bit.html