Another excellent article from the folks at WebUpd8. This has always been a goal of mine.
Lifehacker has a nice post on installing the latest Mac OSX (OSX86) in VirtualBox on Windows, however their instructions work for Linux too - I've tested it in Ubuntu and OSX 10.6 runs smoothly and without any issues.
Remember to use an OSX86 ISO (Lifehacker recommends you download an OSX86 Snow Leopard torrent created by Hazard) and everything will be as easy as installing Ubuntu - you will however need more resources then for a regular Virtual machine, especially for the hard disk space. And of course, you'll also need the latest VirtualBox version.
Note: even though the Lifehacker article mentions that you'll be needing around 1 GB of RAM and that the virtual hard disk size should be around 20GB, you can use less if you don't have enough RAM / HDD space to spare. I've used ~500MB of RAM and a HDD of 11.11 GB to install the latest OSX Snow Leopard in Ubuntu 10.04 Lucid Lynx. But of course, if you want to use Mac OSX Snow Leopard for more than just see it work, you'll need to assign the VM some more system resources.