Dammit! I went to use the Pi and the bloody thing wouldn't power up properly. The power LED lights and the activity LED briefly flickers. A USB meter showed the Pi supply voltage drops to ~3.6V after a short time. Not enough for the Pi to boot.
Maybe I've messed up the wiring and I've got some current going somewhere it shouldn't and something is overheating? I need to pull it apart again to investigate.