Tonight I had to move my VirtualBox guest from one host to another. The guest had a snapshot that it was working from. This caused me a little grief due to ambiguous wording in the Virtual box documentation, and from the look of the forums, others are having a similar problem. So, here's how to do it:

Be very careful that you have enough harddisk space to perform copy operations. If you don't then you will lose you snapshot data. You have been warned. Happened to me :-(

On the current system open VirtualBox Management window. Select the VM you want to move. Click Snapshots. Choose to discard the current snapshot. DISCARD, yes that's right. It doesn't actually delete data, and it should be called "MERGE", because that is what it does. It then merges the snapshot to the base image VDI. Once all the snapshots are merged, you can go to command line, and do:

VBoxManage clonevdi /path/to/vdifile.vdi /path/to/backup.vdi

Usually you will find the original VDI file in /home/username/.VirtualBox/VDI.

Then you can copy the backup.vdi to another machine, store it away, whatever, for adding to a new VM.