layman wrote: Unfortunately the backup did not include my WindowsServer2003 guest
Time Machine was designed to backup documents after creating and after editing.
Time Machine was NOT designed to backup documents while the document is being edited.
Various editors have auto-save feature/facility for backup of the document in the midst of editing, replacing the entire document upon closing.
VirtualBox, like the other virtualisers, is based on the "document template".
When a virtual machine is running or active, it is being edited - the virtual disk file is open and being altered. Time Machine can NOT produce a valid backup of an open document. If the document (virtual disk) has been split into files of less than 2GB, any individual file at any given moment may be closed and Time Machine, during a time slice, may backup that individual file before it is edited with the 'current' data.
While most virtualisers have a snapshot feature/facility, none are a full functional auto-save. They create only a delta update to the virtual disk, not a replacement. The user is required to cause the replacement of the virtual disk by the merging of the snapshot updates with the 'base' virtual disk.
Time Machine backups are not dependable unless the virtual machine has been shutdown.
A backup of saved-state virtual machine is iffy IMHO, as Time Machine may NOT have backed up the full configuration of files comprising a virtual machine during a single time slice.
Copying the folder of a
shutdown virtual machine to another hard disk makes an excellent backup with VBox Version 4.x
I could rescue my MacOS10.6 host from a TimeCapsule - so far so good 
With big efforts I've got the 60GB *.vdi file extracted from the damaged HD. Unfortunately the machine was in action when the crash happened 
