I'm just starting to read this article in the NYT Sunday Magazine from March 2:
http://www.nytimes.com/2008/03/02/ma.../02sex3-t.html
Just started it, so I can't comment intelligently on its contents yet.
In general, I think there's something to be said for the fact that boys and girls learn differently and have different attention spans, and that single-gender learning environments can eliminate some of the gender-difference distractions in a classroom.
The question for me, though, is whether those advantages are large enough or important enough to outweigh the presumed social benefits of learning to navigate in a two-gender world. Will segregation make either boys or girls less able to deal with that world when they grow up?
Plus there's the "does one size fit all?" question. As a matter of statistics, some boys and girls will do better in a mixed environment than in a segregated one, even if in general segregated classes produce better results.