I am in the process of installing the latest debian stable on VirtualBox (3.1.4), and got to an installation step asking to installing GRUB to the MBR of the disk. I assume that since this is a "virtual" disk created by VirtualBox, that the MBR for it is isolated from the MBR of the primary disk on the host. Is this correct? I had previously been under the (possibly incorrect?) impression that there was 1 MBR per physical primary disk.
So it does not affect the host MBR. Only the disk image file.