"...when has definition ever affected pluralization?" As well as the "exocentric" compounds already mentioned, here's another example of a different kind: index in the sense of a serial number (e.g. in a formula or indexed table) has the plural 'indices', but books have alphabetical 'indexes' at the end.
Bringing the Google vote up to date, its now Mice 43,000,000; Mouses 8,070,000; Mouse devices 98,600 and (my favourite till today) Mouse units a mere 1,290. So I guess I'll use mouses in speech (while it's still plausible), but in formal writing I'll have to change from mouse units to mouse devices. Definitely out, though, is Mouse pointers, which is used to mean Cursors.