Apparently the folks at 37-signals -- a Chicago based business thinks it is. They implemented a 4-day work week last summer just for the summer. They determined that productivity didn't decline at all in four days vs five. That people came back to work more refreshed on Monday and worked harder, better and smarter while they were there. I would also speculate that people would spend less time doing personal business during the work week because they can get it all done in the three days on the longer weekend.
"Three-day weekends mean people come back extra refreshed on Monday. Three-day weekends mean people come back happier on Monday. Three-day weekends mean people actually work harder and more efficiently during the four-day work week".
As we start discussing the costs of communiting and traffic flow, there will be more companies that decide that a 4 day work week is the way to go.
Besides, who decided that we should work over 70% of the time anyway?
Hat tip: PSFK