How to easily port VirtualBox machines?

"You can just copy the files inside the directories you mentioned, that is, the virtual harddisk (.vdi) and the xml-based description of the virtual machine. I did this twice, even cross plattform (a XP geust from an OS X host to a Windows XP host, and an Ubuntu guest from a Windows Vista Host to an OS X host) and it worked fine.

There may be two issues:

Both VDIs will have the same UUIDs, which is certainly not how it is intended
You may have to make minor changes to the XML-based machine defintion. I remember that it once contained an absolute path to the vdi, which was not valid after the host-to-host transfer. However, I think newer versions of virtual box do not use absolute paths."