I've suspected this was true for a couple of years now and have been designing websites with this in mind. Research data from OneStat.com indicates that 1024 x 768 is the most popular screen resolution in use by computer users with 1280 x 1024 coming in 2nd and 800 x 600 3rd.

There are some comments on Digg which suggest we should keep designing for 800x600 since many users don't browse the web with the window open to full screen.