From years of consulting I'm always surprised when I feel like I've worked a full day over 8 hours, but am only billing an honest 5-6. Distractions, life responsibilities, break time, etc. all mean 8 hours is not 8 hours. I do think most employers know this and it's acceptable - after all salaried employees are not paid by the hour and being there can be as important for overall productivity as it is for the same person to be individually productive.
Personally I'd be in favor of a 4 day 10 hour work week. I think it could be similarly productive, cuts down on commute time, and yields a better quality of life.
I think the rational solution is to move to a 6-hour workday. You can argue that then people will still spend 2 hours not working, but I think that's not most people's experience.
8 hours doesn't make sense for knowledge workers.
Even if you're in a position like manufacturing or retail, the continued year over year productivity growth since the 8 hour workday became norm should mean that you can enjoy a higher standard of living while working 25% less time. Of course this is not true as workers' compensation has nothing to do with the surplus value extracted from their labor (a fact which works against you if you actually have to be accountable for your work; OTOH if you spend most of the work day goofing off on the Internet or being an incompetent manager, it really works in your favor).
Personally I'd be in favor of a 4 day 10 hour work week. I think it could be similarly productive, cuts down on commute time, and yields a better quality of life.