Think there is a shift up towards 1024x768 but you may still be best to design it at 800x600. Would love it if 1024x768 was the lowest anyone could have but I'm sure there will always be someone who wants something big.
My site was designed at a resolution of 1024x768 however, it may be worth designing at 800x600 to please the masses (and those who trawl the internet but don't actually know anything about PC setup, like how to changethe resolution.)
Many programs will let you design your site using a % feature so that, regardless of the resolution of the end viewer, the page will fill the screen.
So there does seem to be a move to 1024x768 but this seems to be by amateurs for sort of personal/family sites etc. and now that a lot of us have the bigger screens there is also the need to show off to friends and family that we have superior equipment. I know that's laughable but we all know people like this ;o)
Businesses seem to stick with 800x600. For instance I doubt very much if you would find very many Estate Agents who do not use 800x600.