Thought someone might...
Well, it was when I installed a new sound card, I unplugged the front audio header, then plugged it back into to the old card, leaving both cards in. Unfortunately, I had been busy with a file, making some water cooling holes in the case big enough for an external neon switch, thus introducing aluminium particles into the case. The audio header has an AC97 and HD Audio headers on it. I had the HD attached leaving the AC97 out. The aluminium particles built up on the cable driectly beneath the hole and shorted the header while listening to music.
The cables are pretty thin so they got very hot and ignited the insulation. No real damage done, a bit of insultaion tape and a compressed air can sorted it all, but it was on fire and smoke seemed to be coming out of the GPU so I was bricking it.
Moral of this is...make sure you remove any possible shorting material...d'oh
Makes for a great anecdote..."oh yeah my pc caught fire the other week"