Derek Wenmoth, in a post on the DEANZ blog, led me to this report from the US Department of Education.

A U.S. Department of Education analysis of  research studies undertaken between 1996 and 2008 concludes that:

• Blended Learning is the most effective mode of education;
• Online Learning is more effective than face-to-face;
• Face-to-face learning alone is the least effective method of the three.

Note that these findings are confined to tertiary education as the researchers did not find enough studies at the K-12 level.

The full report can be found here:
U.S. Department of Education (2009) “Evaluation of Evidence-Based Practices in Online Learning: A Meta-Analysis and Review of Online Learning Studies” (PDF)