Apple has started pushing out a firmware update for the late-2012 Mac mini, bringing along a fix for an issue that keeps USB keyboards from being recognized when the mini is woken from sleep.
Here's how Apple describes the fix:
This update is recommended for Mac mini (late 2012) models. This update addresses an issue that may prevent a USB keyboard from being recognized after the system wakes from sleep.
The update is fairly small, clocking in at just 4.8MB, and should be available from the Mac App Store, or as a direct download from Apple at the link below.