The last 10 locations you used are stored in a most recently used (MRU) list. To see the list, click the down arrow on the right side of the Location field in the meeting request form, or press the up/down arrows.
It'd be nice if you could clear out the MRU easily. Also it would be nice if it stripped out leading spaces.
It would be very nice indeed if Outlook would cache not only my mail but also the free/busy times of my "usual suspect", i.e., the 10-50 contacts with whom I meet most frequently, as well as my usual locations, e.g., the conference rooms in my building.
Thank you, Outlook team!