I just dismissed the screensaver - why is every keypress ignored?

You just authenticated to your Mac, and the screensaver cleared, to put you right back to where you were - almost.

This is why every keypress is ignored - aside from beeping at you.

Look more closely… Why are the window controls grayed out, as if it's not the active window?

Yeah; it's not exactly right back where you left off - the keyboard focus has not returned to your window; apparently, a side effect of the screensaver. 

Solution: Click where you want the focus (your document, terminal window, whatever), and off you go.

Note: This also affects at least some global key equivalents (I recently whipped up something to really Skip a track in iTunes) - and in such cases the solution is the same: Click somewhere, and they'll work.

Weird.