VirtualBox and Ubuntu 8.04.1

When installing a Ubuntu 8.04.1 virtual guest under VirtualBox 2.06 running on a Mac you will probably be faced with the following error:

Starting up ...
This kernel requires the following features not present on the CPU:
0:6
Unable to boot - please use a kernel appropriate for your CPU.

This problem is due to the fact that the last couple of versions of Ubuntu have been compiled with PAE enabled – but the default guest setting of VirtualBox is that PAE is disabled. So to solve the issue, just stop the virtual guest and enable support in the CPU for PAE/NX, it’s under the advanced general settings. Another solution would be to reinstall the guest using the alternate CD image of Ubuntu (which, last time I checked, didn’t require PAE).