I shot a night high school football game last night and lets just say, The lighting wasn't what I was used to. When I first got there and started to get my camera settings dialed in, I realized that this was going to be a challenge. My initial settings were ISO 6400, F4 and that would only allow a shutter speed of 250-300---not near fast enough for a football game. After about 5 minutes of shooting, I decided to try the high range iso (12800). At this ISO I was very concerned that nothing would be usable. I was pleasnatly surprised. Yes, I did have noise and IQ problems and my shots were certainly not as clean as I had wanted, but I think I still managed to get some fairly useable shots.
Now, I have always been reluctant to go above ISO 1600 and even that is too high for what I try to achieve when I shoot, But after last night I will be more willing to bump the ISO if the shot requires it.
What I learned:
1. If you need the high ISO, use it. (you can fix noise problems in post, but you can't fix motion blur)
2. With this kind of lighting, frame rate and AF is slowed considerably
3. IQ takes a hit with these settings.
4. Better a less-than-perfect shot, than no shot at all.
Here are some examples: 7D, 500L, f4.0, shutter 650, ISO 12800 (of course I used noise reduction in LR3)
More here: http://rwilliamsimaging.com/p35216761#h13a1c4cf