Ah. I'd been wondering the same thing as the OP, as I've installed Debian numerous times on Virtualbox but never seen that option - guess I always took the easy way out rather than go with the 'expert' install.

Somewhat related question... once you have a guest Debian system installed with the virtualbox ose stuff - which is almost always out of date compared to the version of virtualbox being run by the host... whats the best/recommended way to get rid of the 'old' and set up with the new/current guest additions? Last time I tried (and its been a while), it seemed like it wasn't exactly obvious - apt didn't want to get rid of the packages, etc.

Try using synaptic click on package you want to remove select remove all