I think I've discovered the cause for more sore neck/shoulders these past few days: I just noticed that the monitor was 2 inches higher than it normally is.
Happened while I was dusting out the snake pit of wires behind it, a few days ago. The screen slides up and down at the slightest touch. It is really well-designed and very cool, but it didn't occur to me that I'd be screwing myself up so badly just by bumping the monitor.
Still, it does makes perfect sense to me, that holding my head at a different (and bad) angle for hours on end would cause muscles to spaz out. I'm such a moron sometimes.
Hope that's it.