This picture was taken at 1/160s at f4 with the camera set to ISO6400. A few years ago those camera settings would have resulted in horrible picture quality, making low-light photography very difficult.
Now, with cameras like the Canon 1D MkIV (or the Nikon D3s if you shoot for the "other" side), shooting at these super high ISO settings results in really good quality files, allowing available light shooting in even very dark conditions. This shot is almost all available light, with a tiny bit of flash added just to add a bit of fill and improve the quality of light on the faces. If I'd shot at a lower ISO very little of the background would be visible - I'd have ended up with three correctly exposed people dancing in the middle of a black hole.
The dancing is one of the few occasions when I use a flashgun - and only then if I have to. Sometimes the lighting for the dancing is so dark I can barely see what I'm shooting and its those times when I'm thankful for fast and accurate autofocus.
Don't think though, that its the camera that does all the work. Yes, the newest generation of digital cameras can help the photographer to get pictures which would otherwise be impossible, but ultimately its the photographer that chooses where to stand, what to include in the frame, when to press the shutter release, and which pictures make the final edit for the album.
I read a great quote the other day which sums this up nicely...
"Telling a photographer that his camera takes great pictures is like telling a chef that his oven cooks great meals".