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^ I can't speak for Will, but I'd rather use ambient light than flash in almost any situation. The point at which I switch to flash varies with light, but on most receptions if I can AF at 6400 while holding 1/125 at f/2, I'll go with ambient. And regarding the shutter speed he used, that's right around where I need to be to have a decent chance of stopping motion on the dancefloor.
Here's a shot from our most recently blogged wedding. 10,000, f/2, 1/50. I almost never deliver anything with motion blur, but I liked the subject, lighting, and composition enough here to keep this one. However, if I'd taken this at 25,000, that would have given me another stop of shutter speed to work with. But I have trouble generating keepers at that ISO with the D700; 12,800 is about my limit on most weddings.