Ryan Lee Carson of Treehouse:

We work a 4-day week (M-Th, 9-6) because we think that information work isn’t like manufacturing. Another hour at the MacBook won’t yield another $1,000 in profit. We believe that smart folks can get five days of work done in four days. Simple as that. 

Bravo.

Services (consulting, contracting, etc.) aside, this could not be more true. 37signals famously does this during the summer as well, and finds that their productivity remains the same.