More on CSS Selector Performance
Published by Chris Coyier
This is a direct link to a PowerPoint file by Steve Souders (in April 2009). Of particular interest is pages 19-33 with all the information on CSS selectors. Evidence points to yes, there are efficiency differences in how you write selectors. But efficient CSS can sometimes come at the cost of larger CSS (bad) and that a "real world" levels even "costly" selectors aren't that big of a deal.