That's the problem with vacation, and why, at least for many people, it isn't as relaxing as it should be. I'm sure there are jobs at there that do this, but from people I've spoken to, and my own experiences, as soon as you leave there is no one to pick up the slack and help lighten the burden in your absence. I have friends who hate taking real vacations and pretty much use their vacation time to just extend a couple weekends a year just because they know if they take a week off they'll come back to more work than they had before.
I'm curious how this is for the non-US people. Because thinking about it I have family outside the country who don't have this problem. When I was visiting my family in Trinidad my uncle took a full month off for vacation and went back to work in the latter part of my stay without any extra work. In fact all of my family down there do this (and they all get a full month of paid vacation a year). I think I need to move down there god dammit.