I saw someone else mention this, but I've lost the attribution.  Anyway,
I too have been seeing "shutdown -r now" hang, and upon seeing someone say
that killing lpd appears to be the culprit, I tried it too.  System became
unusable almost immediately.  (I got a new shell prompt, but even hitting
carriage return failed to get a response.)

My default printer, for which lpd runs, is a remote print server, so this
isn't a serial port (or parallel port) lockup thing.