This is Virtualbox 4.1.10. The host is 64-bit, the guest is 64-bit, and the OS type is set to "Linux - Debian (64-bit)".
Also, there's no HDD activity, and no notable amount of CPU time is being eaten. However, the guest is not "frozen". In fact, it's responsive as normally: the progressbar is still animating, and I can even open applications.