I've been running OS X Lion since the update was available and encountered no problems with my Microsoft USB keyboard. Until today, that is. For whatever reason, my Mac refused to recognize the keyboard, even when I changed the USB slot it was connected to.
The solution? I shut down the computer and removed all USB connections (i.e., keyboard, mouse and printer) and then reconnected them all in different ports. After re-booting, the Mac was able to recognize the Microsoft keyboard and functionality was restored.
