Since Yosemite is now Generally Available, I decided to download it and install it on my Late 2013 Mac Book Pro Retina, 15 inch edition.
That was a mistake.
My USB Keyboard no longer works; and given the internet results, it seems that’s a common problem.
I guess I need to treat Mac OSX releases like I do Windows releases: Always wait for SP1.
I’ll update this post as I know more.
Edit: I’ve tried the steps in this content farm article to reset the System Management Controller, No Dice.
Edit 2: After resetting the SMC and walking away from my computer for 20 minutes (enough time for it to be asleep for 10), I woke it out of sleep and it recognized the keyboard again.
It’s unclear whether resetting the SMC is a requirement; but it does work now.
For reference, I have a Microsoft Natural Keyboard.