Friends of mine complained about OpenSuse 11.1 in VirtualBox: they were simply unable to compile the guest additions. I ascribed these problems to their inexperience. I was wrong. :/
Novell preinstalls the guest additions, and of course an ancient version! That's what causing all these irregular error messages, and all those frustrating installation experiences. Do a 'zypper search virtual', and remove everything which looks like one of these Suse-attempts-to-be-helpful-packages. Enjoy!
Having bypassed these difficulties, I've now successfully completed the transfer of all of my virtual machines from VMWare to VirtualBox. All (Arch, Debian, Ubuntu, Suse, XP, 7), except for the legacy machines like Windows NT and 98. Those will stay forever in VMWare. Naturally. :)