I don't think there's a standard at the moment, it all depends
I prefer to make mine 1024 as not many at 800x600 resolution
However, its always nice to give the option, or some use fluid (though I have never been a fan of that!)
90% I like using fluid when I can. When I can't, somewhere right around 900, depending on the site.
There are some people who sill use 800x600 but I'm guessing they do it for accessibility reasons. Older people or people with vision related problems that can't read small text. Even so, they usually have something that helps them at larger resolutions.
I am starting to see a much larger number of 1280x1024 in my analytics, but 1024x768 seems to be the most common.
I usually keep my designs at between 780px and 980px, depends on the site really. I rarely use fluid widths...with the more widespread use of 16x9/16x10 monitors, I think fully fluid sites on these WXGA screens look awful (unless there's a max-width set) and the readability of the site suffers.