I have a nikon d80 and am looking to buy a telephoto lens to take sport photography. (eg surfing and rugby). I'v been looking at the Sigma AF 50-500mm f4-6.3 EX, is this a good choice, if not any advice?
thanks.

I don't know if the BigMa will be fast enough. It may be a good idea to look in the sports photography forum and if the q isn't already there then post your question there.

Nature and Wildlife folks use long lenses, too.

Depends on how much you want to spend, I hear the Nikkor 300mm 2.8 AF-S is a pretty damn solid lens :)

I'm going to save you a lot of money by telling you that the sigma telephotos aren't great :(. There's only two sigmas I would buy (and they are REALLY good) but their other lenses are average to fair. You'll be much better off buying a Nikkor 300mm f/2.8 for about $200 more. The sigma needs A LOT of daylight so on cloudy days you will not really be able to take good action shots because of the longer exposures needed. The Nikkor 300mm is much more solidly built, has a max 2.8 apperture (more light comes through the lens) and has much higher sharpness due to its telephoto prime construction. I would highly suggest the 300mm f/2.8 completely over the Sigma.

I'll have to go a few steps back, fx101, I wouldn't discount ALL sigma telephotos. Their 70-200mm f/2.8, 50-150mm f/2.8, 100-300mm f/4, 120-300mm f/2.8, 300mm f/2.8, and 500mm f/4.5 are all superb optics, with the 100-300 f/4 reportedly even besting the Nikon 70-200mm in their common range at f/4, and the 120-300 offering a compelling lens offering at a good price.
I'm going to agree though that the 50-500 is not very good. I have tried one and I was not impressed. The slow max aperture makes it tough to use in anything but daylight.