Apparently you can only run Mac OSX in VirtualBox if your CPU supports hardware virtualization (VTx / AMD-v).

You can find out if your CPU supports this by running the following command in a terminal:

egrep '(vmx|svm)' /proc/cpuinfo

If the above command returns "vmx" (Intel) or "svm" (AMD) among the other results, then your CPU supports hardware acceleration. If not, see if you have an option in the BIOS to enable Hardware Virtualization (VTx / AMD-v). If there isn't any... you can't run Mac OSX in VirtualBox.

Read more about virtualization @ Wikipedia.

Many thanks to UbuntuLife for this tip!