From what I can tell, VirtualBox does not have a 64-bit binary for the Mac, and VMWare does not provide a Player build for MacOS. Are there any virtualization options for running a 64-bit guest OS on a Mac host?
VirtualBox will run 64bit guest operating systems on MacOS X. See http://www.virtualbox.org/manual/ch03.html for more info.
VMware Fusion 3 will run 64-bit guest operating systems. This link is a preconfigured search on vmware.com for 64-bit Linux guest OSes qualified to run on Fusion 3.1 http://bit.ly/fxwh5C (here is a secondary list that might be more current, also from vmware.com http://bit.ly/dIFImt )
Concerning the host OS, this article from vmware.com talks about compatibility of Fusion running with either a 32- or 64-bit kernel on Mac OS X. The version of the vmware-vmx process will be a 64-bit process if you're running in a 64-bit kernel, and a 32-bit process otherwise: http://bit.ly/hsUfDw
Modern Macs boot with a 64-bit kernel by default, and Snow Leopard is 64-bit OS. If your concern is in regards changing the kernel (32- or 64-bit) that your Mac host OS has booted with, you can adjust that per Apple's KB http://support.apple.com/kb/ht3773
Here are the release notes for Fusion 3.1.2 http://www.vmware.com/support/fusion3/doc/releasenotes_fusion_312.html