According to the PC Dictionary it is "mice." For your reference: PC Dictionary. However it has also been said that because it is a name it is "mouses."

Without a doubt the plural of computer mouse is: MOUSES

MOUSE is an acronym, and is short for Manually-Operated User-Selection Equipment so technically MOUSE is already the plural (... Equipments?) Most people, however pluralise acronyms, and initials in general, by adding a small s, so it would be MOUSEs, which seems a bit silly. Mice is also wrong, as it doesn't stand for anything, but it does make more grammatical sense than calling them MOUSE or MOUSEs. (NB: If a MOUSE was so called because it looked like the animal, then mice would be correct, but that isn't the case, although small body, long tail, who knows...)

http://wiki.answers.com/Q/What_is_the_plural_of_'mouse'_on_a_computer