I design for 1024*768. This time last year 48.3% of people used that resolution (
http://answers.google.com/answers/threadview?id=341264)
Alot of sites say that 800x600 accounts for over 60%, but this is outdated info based on outdated sources.
No doubt the 1024 % is even higher this year, as it is the standard new laptops are set at, and new monitors are made to display even higher than that.