This was a topic of discussion (in European design blogs) about a year ago. As a result Thomas Baekdal did a reasonable multi-site study and published his findings.
We know that visitors OS, monitor size, and browser version vary widely by niche. As do open sidebars, toolbars, etc. I strongly recommend that concerned webmasters develop and run a site viewport analysis on a recurring basis, i.e. for the same month each year - it doesn't need to be continual, simply able to notice significant change. Certainly a potential competitive design to conversion advantage.