We're doing 4-day weeks in the summer. They work better there since a good portion of the company lives in cold-weather climates. The extra day was often used for working, and when it wasn't it made the week feel like 3.5 day weeks. So we're starting to think of it as summer vacation every work week.
That's the current idea. We're ever-chaging so we'll see how it all shakes out.
Jason, care to comment if you see this/get a chance?