I gave this a try with a USB mouse I happened to have on hand a couple weeks ago and was successful:
But it might well be that only specific USB mice have the PS/2 compatibility needed to do the simple direct interfacing with an Arduino board.
This was my first attempt with a mouse, so I'm one for one on success.
With keyboards, I had success with all three I had on hand, and one tenkey, but struck out with the other super cheapo tenkey.
Still, pretty impressive that there are still so many devices with this compatibility mode with such an old interface.