It depends on the lighting, and the type of shot, but I have used ISO 1600 before and you can hardly see the noise. Some 3200 shots look really good too.

12800 though, eurgh!!!

Because I shoot a lot of available light photographs outdoors I sometimes shoot at ISO 1600. Waterfalls, for example, will always have a shady setting just because they're formed by running over a cliff. Most are in shade so a high ISI and tripod are a must to get slow shutter speeds to blur the falling water. Since getting my D90 I find I'm shooting at least at 800 ISO just to give myself a wider range of exposure latitude for better depth of field control. I always shoot in Aperture priority.

I agree with Steve here, I normally shoot at 100 or 200 ISO, unless I am photographing dogs racing then I will bump it up to 400. I rarely go above this although on severely overcast days I might venture up to 800 if I need the shutter speed (again at rallies - all 'stills' are taken at a much lower ISO than this).

I think it also depends on your camera - mine is ok up to 800 ISO but at 1600 the noise is noticable. The last time I shot at ISO 1600 was at Crufts in the BIS ring as I needed the shutter speed. The photo's came out fine but the ones I gave to others I passed through the noise reduction software first.