As many point out if you stick an Apple sticker on your PC you're probably covered. The EULA for the workstation version of Leopard prevents it from being run under emulation and as a result there's no support in VMWare for this.
The first route requires modifying (or using a pre-modified) image of Leopard that can be installed on a regular PC.
Update: You cannot create a Mac OS X Client virtual machine for OS X 10.6 and earlier.
Apple does not allow these Client OSes to be virtualized.
If you're running an AMD machine or something without SSE3 it gets a little more involved.
If you purchase (or already own) a version of Leopard then this is a gray area since the Leopard EULA states you may only run it on an "Apple Labeled" machine.
We're looking for long answers that provide some explanation and context.