My company is moving to a new location.  Because of a union manuever,
some employees with seniority who have window offices in the current
location will lose their window offices in the new location.  I am one
of these employees.  I desperately want to come up with a decorating
idea for a windowless 15' by 15' office with one standard door in one
corner. I want to make it as desirable as any office in the building,
even those larger or with windows.  It is a matter of principle and a
matter of pride.  I will not be satisfied with the obvious decorating
ideas.  I want something creative and inventive that no one has
thought of and something that will not make permanent changes to the
structure.  We lease the building.

Hi jandea,
 
Here's a few tips for decorating a windowless office from Inc Magazine: 

Space Centered
http://www.inc.com/magazine/20030901/design.html

"Location, location, location: Desks should be placed in an "armchair
configuration": Your back should be to the wall, with plants or
cabinets on either side of you to create a safe, protective
environment.

Let the outside in: A feng shui principle equates water with money --
a fountain or aquarium in your office fosters prosperity and wealth.
Plants symbolize life and growth.

Assume the position: Never sit in front of or with your back to a door
or window, which leaves you exposed and vulnerable. Have a clear view
of both, but without being in direct line of either. If your back is
to the door, place a small mirror to reflect anyone entering your
office. If your back is to a window, place something on the ledge or
sit in a high-backed chair. No windows? Mirrors help, even if they'll
never beat a view."

[edit]

"Offices should maintain a palette of colors from the elements of fire
(red), earth (orange or yellow), water (blue), wood (green or brown),
and metal (white or gray). A rainbow of Pendaflex files won't cut it,
but walls and furniture in these colors will."

Best regards,
tlspiegel

Sounds like a jail.

Why not put bars along one side and a picture of Rita Hayworth on another?

(In case you didn't know, this is an allusion to The Shawshank Redemption.)

If you have the budget for it, and the building owners will permit it,
a large, well-maintained aquarium can turn a plain office into the
most interesting office imaginable. Not only are the undulating plants
and swimming fish fascinating to watch, but the expanse of glass gives
a window-like extra dimension to the room.

Ouch...an office with no windows.  This one is a challenge.   
It seems as if it is important to you to have a central focus on
something.  Perhaps you might be able to commission an artist to paint
a mural on the wall.   This would not only add a theme - of your
choice - but it would also be very unique for a 15x15 office.   I have
seen a few offices that I have like using fabric on one of the walls. 
 Whether tiling the fabric with panels or just being creative somehow.
  Anyway, goodluck with whatever you do.

Create a conversation piece and decorate at the same time. Paint a
picture of a window revealing a bright sunny day outside.