First of all what is screen resolution?
resolution - the maximum number of pixels that can be displayed on a monitor, expressed as (number of horizontal pixels) x (number of vertical pixels), i.e., 1024x768. The ratio of horizontal to vertical resolution is usually 4:3, the same as that of conventional television sets. (Free Online Dictionary of Computing - http://foldoc.org/)
Following statistics has been provided by StudentsMetro.com and DataChild.ca for 2008, with combined traffic over 500 000 visitors (gathered by Google Analytics)
StudentsMetro.com:
DataChild.ca:
For me these results were surprising, but as you can see most popular resolution is 1024x768. There are some differences in the percentages, but this is caused by different audiences on both sites, StudentsMetro.com targets general public, when DataChild.ca is targeting specific group of people who are playing PoxNora (online strategy game).