virtualbox.org

I guess you could call that migrating physical machine to guest (the reverse of this).

You know, instead of installing the guest from scratch (in cases where it would be the same system as the host anyway).

It's probably a dumb question but I had to ask...

Asked and answered many times (and the answer is "yes"). Try a google search for P2V (physical to virtual) using site:forums.virtualbox.org.

In general any migration, whether P2P, P2V, V2P, or V2V (the latter between different VM platforms, or the same platform with different hardware settings) raises exactly the same issues, namely compatibility with and/or drivers for different hardware, and activation issues for Windows ports.