After suspend/resume, the HDMI port does not show up, but it can still be used (if you use e g pavucontrol to select it). Sometimes, doing seemingly unrelated things, such as running the alsa-info script, restarting PulseAudio, etc can make it show up again.
Found on both NVidia and Intel hardware.
Fixed upstream by http://git.kernel.org/?p=linux/kernel/git/tiwai/sound.git;a=commit;h=8a5354140a86b6d4057793a9ed28d29ac8ce6ba6
Which kernels should we consider backporting to?
* upstream stable