When I first read about the uber-high ISOs I thought "why would anyone ever need that?". I now stand corrected. I was shooting a rock concert, and the lighting was nothing short of terrible (lighting guy admitted that himself when I struck up a conversation), and I had to digitally boost my images to even over ISO 256.000!
Picture proof, this was with a F/2.8 fisheye at 1/50th of a second (any lower would really blur the performer's motion):
There was also some TERRIBLE backlighting aimed directly into the eyes of the crowd, which apparently killed my D90's metering (all modes) and made my 2.8 fish flare like madness:
Still got some acceptable shots out of it but man, I got stressed (and I, of course, could not use flash)....