I missed this when it came out last May but ran acrross a link to it alongside the Science Daily* article that led to the previous post. A longitudinal study of 1800 kids in pre-kindergarten has shown that kids' language ability grows faster if they are in a preschool with kids with better language skills than their own, and this is even more true for kids who start with good language than for those who don't. It's also more true in well managed classrooms.
This is an argument for mixing CSPP or Head Start and full-pay kids in the same classroom. It's also an argument for not doing that, depending on whether you want to help the kids who are starting higher or the ones who are starting lower.
*I look at Science Daily a couple of times a day, so you don't have to, but it is a wonderful site if you have a few minutes to browse. They have a child development section where I find a lot of what I post here, but if your scientific interest goes beyond child development or beyond what I post here, this is a good site to visit regularly.