Google does something like this, on a selective basis.
I think it started as something done only for special cases, but I know a few people who arranged it. One woman I know works three days per week instead of five, for 60% of her normal salary. She has also taken a large chunk of her 18-week maternity leave and uses it one day per week, so she actually works two days per week but gets paid for three, until the maternity leave runs out. Her husband has arranged a similar structure with his employer (not Google), working three days per week so one of them is always home with the kids. She's a fairly special case, though, because she's a freakishly brilliant software engineer who any smart company would bend over backwards to accommodate.
However, it's now been expanded to be made generally available to full-time employees. It requires management approval, but the descriptions I've seen make it clear that management is expected to agree unless there are specific reasons why it can't work. Salary, bonuses and stock are pro-rated based on the percentage of a normal schedule that is worked. Most commonly, people work 60% or 80% schedules (i.e. three or four days per week instead of five). Other benefits, such as health care, etc., are not pro-rated, but either provided or not, depending on the percentage of normal hours worked.
I could see myself going to a 60% work week in a few years, having a four-day weekend every week in exchange for a 40% pay cut.