There's a guy running a newsletter that encourages freelancers to take time off, and gives answers to counter why you don't: http://somedaysyndrome.com/

Apparently you're by far not the the only self-employed person who doesn't like to take time off. I will say that for me, it's been utterly amazing to realize that if I don't work, I can still get paid (up to a point). Still never GO anywhere. But the house is kinda our vacation for a bit.