From the reading I've done, Time Machine does make and keep local backups so they are readily available should you not have access to the "external" backup drive.
I'm sure in the future the Time Machine preferences will include more configuration options, so until then I have chosen to disable the local instance of the backup. Since it looks like you backup to an external drive that is connected directly to your machine, you can probably get by with this as well. Myself, I rarely need to do restores and the connection to our time capsule server is very fast, so I think it will be the solution I stick with.
Here is the article explaining how to disable and re-enable the local backups on OS X Lion: http://osxdaily.com/2011/09/28/disable-time-machine-local-backups-in-mac-os-x-li on/
Good luck, Solaar!
I believe that once the service is disabled, they expire and purge on their own...
Looks like they should be removed automatically: http://support.apple.com/kb/HT4878
Local snapshots are periodically condensed into daily or weekly snapshots to minimize the space used on your disk. If your disk is low on space, Time Machine stops creating new snapshots, and some or all existing snapshots may be removed to make space available for applications to use. If sufficient disk space becomes available again, Time Machine resumes creating local snapshots. This means your disk will have the same amount of available space as it would if Time Machine were not enabled.
Message was edited by: emeacham