Modern bikes are set up from the factory with very specific geometry in mind. The most important factor in all of this is called 'trail' and the trail must not be altered, lest your ancy fun-to-ride bike might buck you off and onto a fire hydrant.

Wheel diameter (including tire size) is one of the dimensions that trail affects... so if you alter the tire size you must accomodate by altering something else... decreasing tire size will decrease trail slightly, so you need a corresponding change somewhere else. THe easiest thing to change is head angle (also affects trail)... to increase trail you need to have a shallower head angle, which can be accomplished by using a smaller rear tire. If you change from a 700 X 26 tire to a 700 X 25 front tire you must change the back to a 700 X 23 to change your head angle slightly. If you change your front tire from 700 X 26 to 700 X 23 you must change your rear tire to a 700 X 18.

I only say this because i care about your safety.