Anyone who's not an entrepreneur will have at least vacation. Then you can get totally offline if you want to, as you don't even have to use the computer/internet at work.

Although I prefer just taking a few days off the computer at a time myself. Like a weekend, or not hacking anything in the evenings for a week or so. For me, the problem isn't being online per se but the balance of being online AND offline.

When I'm offline, it will eventually energize me to get back online for a while. And when I'm online, it will do SO good to cut off the lines for a few days when I find myself only being online.