If you had a dual-boot system with Windows (Bootcamp) and Mac OS X, you could run the Bootcamp partition under VMware Fusion for most things, and reboot into native Windows if necessary (for some games).
Actually did something similar back in 2000. We built a dual-processor Pentium II machine and installed Win2k and Redhat (IIRC), then with vmware and a bit of hacking we were able to make each one VM bootable inside the other. Just for the sake of it really. Fun :)
Also, one caveat is that the virtualbox vm write tops out somewhere around 100-120 mbyte/s (idr), while the boot from linux gives me 360.
sudo VBoxManage internalcommands createrawvmdk -filename ~myuser/Library/VirtualBox/VDI/raw-linux.vmdk -rawdisk /dev/disk1
sudo chmod 666 /dev/disk1*