Probably not important, but I'm using Fedora 16 for the moment, and since I was running VirtualBox 4.1 I had to remove the previous package first and install the 4.2 version. This will not destroy the guest hosts.
rpm -e VirtualBox-4.1 rpm -ip VirtualBox-4.2-4.2.0_80737_fedora16-1.x86_64.rpmAfter a new start with the VirtualBox Manager the old VM guests are in place.