I have been using this: http://alex.csgraf.de/self/?part/projects&folder/Qemu%20... in a triple booted MacBook for some low-level testing on OS X, just to see how it works following the Mac OS X Internals book, the performance is passable, I use virtualization because Apple don't have an ADC Hardware Purchase Program for Brazil and Apple hardware are really expensive here.
I'm trying to do it right now, I'll let you know when I've succeeded. Unfortunately, the packaging is in rpm format and my distro doesn't support it (well, I can install rpm, but it appears to be broken, so I have to hack around).
Might have something to do with how "heavy" OSX's graphics are, to the extent it really requires graphics acceleration to just draw normal windows. VMWare can't pass that through to the graphics card in the correct way, so it's really, really slow.
I'm pretty interested in running OSX in a VMWare container for some command line operations, though. I would like to use OSX to do some media manipulation and webkit work that would be an ultra-pain on Linux but is fairly simple on OSX, but I want it on "normal" hardware, so I'll be looking into stuff like this (when it's shaken down a bit more ..)