Even on a long-haul - unless you're really pushing the aircraft's range limits, it's rare for the fuel tanks to be full. You will usually reach your MTOM due to the payload carried before fuel-voulume becomes limiting.
As such, the centre tank will always be filled last, even on short-haul. As OPNL/Starlionblue said, this is a structural requirement to minimise the bending moment of the wings, particularly in the initial stages of flight when the aircraft is haviest. A similar rationale is behind the max ZFM, which I believe is set with reference to the stresses on the wing/body joints, again too much of mass in the fuselage which then has to be supported by the wing/body joints puts additional stess on the aircraft, which is not healthy for them long-term.
As an example of the use of the centre tank, on the 757, for flights up to around 2-2h30 duration the centre tank will be empty - aside from the unuseable fuel which is mandated by the current ADs.
[Edited 2004-07-07 15:20:33]