These are a bit old, but they illustrate some high-ISO extremes so I thought they'd fit in this thread.
Camera is the K-x. Both pix were taken with the FA 50 1.4.
The first picture was exposed more or less accurately for the stage, albeit from a distance, just to give you an idea of how dim things were that night:
This second photo from about the same vantage point and time was taken with the FA 50 wide open at f1.4 at 1/30s @ISO 12800, handheld:
The second pic is quite grainy but it still surprises me that I was able to get any decent image at all just from the ambient light of stage lighting, a remote streetlight, and the flouro sticks people were holding.