Even reformatting a card used in one camera in another camera will not clear the file numbering. The only way to make sure the card is completly clean before swithing it to another camera is to fromat it in your computer first.
I have five different Canon digital cameras, D60, 20D, 40D (enroute from B&H), a G5 and an A70. Each camera has (and will, in the case of the 40D) it's own dedicated set of media cards that are not used in the other cameras.
Even though my downloading program will assign unique file names to my images, based on which camera they were taken with, I don't switch the cards because I want the file numbering for each camera to be progressive. It helps me keep track of the approximate number of actuations for each camera.