In my last but one post I mentioned the importance of meta-reviews. Just came across a preview of a great one on the Columbia University Teachers College site, here (excellent site by the way -- well worth signing up for).
Camilli, G., Vargas, S., Ryan, S. Barnett, W.S. (2010, forthcoming) Meta-analysis of the effects of early education interventions on cognitive and social development, Teachers College Record, 112, 3.
The authors reviewed 123 comparative studies of early childhood interventions, and end with recommendations for future studies and the design of early childhood interventions. See also Steve Barnett's short & powerful webcast on The Voice at this site.