In my life, I don't think there's a significant difference in the amount of work I accomplish during a 40 hour week, vs. a 32 hour week. You hear about companies allowing employees to work four, 10-hour days (and I've even heard of four 9-hour days) because they find out there isn't a difference in productivity. Some of this is probably due to the fact that Mondays and Fridays often end up with large blocks of time wasted, whether it be people trying to psyche themselves back up for the weekly grind, or people counting down the hours, having already checked-out mentally, and just waiting to get out of the office physically.
He's listed some good practical tips that he's using as he's switched over - avoid IM, only check e-mail twice a day, stick to what matters, ask for alone time, limit blog-reading time, make lists, and restrict meetings.
I already do most of these things - I've written about how distracting email is, I don't really socialize at work (which keeps people from dropping into my office), I make to-do lists on legal pads, and I try to dodge meetings. The big sink for me is probably blog-reading - I'll start reading some programming or design stuff, and before I know it, I've lost an hour of my day.
The author's underlying reasons for the change are similar to why I work a 32 hour week as well - "when you work less, it gives you more time to experience life and think."