Well, it's been a good three and a half years, and my watercooling system has finally carked it.
I found it funny that my computer was off when I got home. I turned it back on, but a few seconds into windows it turned itself off again. I pondered for a bit and then checked the water flow - there was none!
The pump was still vibrating, which meant that either the pump's impeller had broken, or there was a blockage somewhere. closer inspection revealed a disgusting 6-8 inch mass in the tube leading up to the cpu block. I've cleaned bathroom downpipes of 5 years of waste build-up, and this is almost on that level.
I can only surmise that a layer of gunk must have slowly built up on the inside of the tubing, and just recently decided to separate from the tubing and collapse into a wrinkly folded heap and jammed up the water flow path. It is absolutely disgusting to look at, and I am not even going to entertain the thought of clearing it, especially when it's fallen into the cpu block itself.
That it took nearly an hour just to negotiate the tubing and waterblocks enough to leverage them out of the way to put the original heatsinks back on is enough for me to consider watercooling as an interesting hobby that in my case at least is best left as such. Granted 3.5 years is a long time for 24/7 operation, but given that computer parts eventually do fail, I would certainly not like to face the inconvenience I'm facing now of having to remove the tubing and waterblocks from my rig without bathing everything in a fluorescent green glow.
I've built a fair few computers for work recently, all based on the Intel core 2 duo platform, and I have to say they really are quiet enough to not be annoying, even for someone as picky as me. So while as a hobby I don't think the excitement of watercooling will be surpassed, at the end of the day the simplest and most elegant solutions wins, and one that doesn't involve going to fetch some unused towels to prepare to mop up the mess....