Yes, I work part-time essentially all the time these days. (I run a small software business and consult on the side, average 20 hours or so a week, and take off pretty much any day I feel like.) If I wanted to, I could sustain this lifestyle indefinitely.

I'm told that many consultants do something like this. If full-utilization for a consultant is $200,000 (and that is by no means a ceiling), then 50% still puts your household in a pretty decent place to be. There exist some consulting opportunities where you can schedule arbitrary amounts of work over long time scales -- I have one client relationship where, if I wanted to bill four hours on Monday and four hours on Tuesday and then never touch the keyboard again, he'd be thrilled by that. There are other opportunities if you want to parachute in, work for two weeks, and then get back to living.