I've done quite a lot of club photography, and a fair deal in other dark places like theatres.

Personally I'd recommend not using a flash. A flash completely changes the atmosphere: yes, flashes let you capture the people, instantaneously as they were, (probably posing for your camera...), but you won't capture how it felt to be there.

Shooting without a flash is difficult. You need steady hands, a camera with decent low light performance (probably a CMOS sensor rather than CCD) the D90 is great in this respect, plenty of patience, and you need to be prepared to deal with noisy images afterwards.

Some tips:

• get to know your camera really well: in particular how to adjust exposure compensation and ISO as quickly as possible in the dark when you can't see anything. (On the D90 get used to using the menu buttons as modifiers for the adjustment wheel without having to look to see which one you want). Also, get used to using the focus lock button!

• practise using all the available light: try taking photos on the street at night, using only the illumination from street lights. In very low light you can still easily take sharp pictures if the subject is in silhouette against a light source, for example.

• don't fight the dark Your photos are going to be dark, because the club was dark! Don't fight this, but make sure that there is still interesting detail: very rarely can you fill the frame with light.

• experiment, and try lots of things: Don't just take the same sort of photo all night: move around and capture as lots of angles and subjects

• use a wideangle lens, and learn to shoot without looking: if you don't have to have your eye to the viewfinder you can capture lots more angles, and catch things faster than if your camera is glued to your face.

• enjoy yourself: It's no fun taking photos if you're not taking part in the party :)

(P.S. my favourite lens for this -- and most other things, to be honest -- is the 10.5 DX fisheye ;)