I found that following these instructions resulted in ALL of my backups being deleted. Perhaps there's a bug or unclear wording in Time Machine.
After starting Time Machine, and selecting the oldest backup folder, the Gears menu displayed "Delete All Backups of ‘2008-01-13-205847′". When I choose to continue, Time Machine displayed the dialog ‘Delete All Backups of ‘2008-01-13-205847′ with a progress bar, and removed EVERY backup on the system. The dialog always showed the January 13 date, but I could see outside of Time Machine that all backup folders were being removed one by one. Eventually every backup was deleted, except for the most recent.
We did some additional experimentation (on a different machine) and navigate to a sub-folder such as "Downloads" within a particular backup. When we choose "Delete Al Backups of ‘Dowloads'", Time Machine delete all Downloads folders from EVERY backup. This was the behavior that we expected, based on the message.
We found that it is safest to delete an old backup from Finder (outside of Time Machine), drag the folder to the Trash, and empty the Trash. When we went back into Time Machine after deleting the backup, the backup was no longer available as expected.

This behavior was noted with Leopard 10.5.5.

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Thank you!

I just followed your instructions and it worked perfectly.

It would have taken me a while to discover how to delete an object from Time Machine without this hint.

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