I was a goofball when I installed VirtualBox. I forgot to disable backups of my VM's. Some might want their Virtual Machines backed up, but as they're mostly just test environments, I consider them disposable. Anything important in the VM gets replicated and synced through Subversion.
The problem is when my virtual machines change, Time Machine suddenly tries to back up the entire virtual drive, gobbling as many as a few gigs of data as one file, and seriously cramping my old backups. Once I told Time Machine not to back up the VirtualBox directory, the old backups (hogging many gigs of backup space) remained on my external hard drive. Trying to get rid of them from the command-line wasn't working, as OS X puts some kind of restrictions on the file system.
The filesystem layout is pretty nifty, as shown below:
Chimera:chimera axon$ cd /Volumes/Time\ Machine\ Backups/Backups.backupdb/chimera/
Chimera:chimera axon$ ls -la
drwxr-xr-x@ 10 root staff 374 Sep 9 22:03 .
drwxr-xr-x+ 3 root staff 102 Jul 29 19:53 ..
drwxr-xr-x@ 3 root staff 204 Jul 5 18:52 2008-07-05-185224
drwxr-xr-x@ 3 root staff 204 Jul 29 22:00 2008-07-29-220053
drwxr-xr-x@ 3 root staff 204 Aug 5 21:50 2008-08-05-215007
drwxr-xr-x@ 3 root staff 204 Aug 30 16:53 2008-08-30-165311
drwxr-xr-x@ 3 root staff 204 Sep 5 23:26 2008-09-05-232659
drwxr-xr-x@ 3 root staff 204 Sep 6 11:42 2008-09-06-114242
drwxr-xr-x@ 3 root staff 204 Sep 6 21:38 2008-09-06-213829
drwxr-xr-x@ 3 root staff 204 Sep 9 22:03 2008-09-09-220316
lrwxr-xr-x 1 root staff 17 Sep 9 22:03 Latest -> 2008-09-09-220316
We won't get too deep into that, though, because it doesn't matter. The answer, oddly, was staring me right in the face... in the graphical Interface... The "Delete All Backups" option shows up under the gear menu, but only when you're browsing your time machine backups.