Ddg4005,
Does you work company have a global presence with demanding customers? I do and our customers do not want to wait 16 to 48 hours for a reply to an urgent e-mail. Sometimes the folks who cover for me when I am away can't deal with a particular issue since they are not directly involved. In those rare cases, I have to deal with the issue myself.
Since I am a salaried engineer, I get don't get paid by the hour so I'm not on the clock so to speak. I get paid the same for one hour of work a week or forty. The flexibility in my job I am afforded far outweighs the occassional need to work for an hour or two while on vacation. I've taken vacations off the grid and haven't had any issues either.
The problem that I see is that some people think they should be able to drop their responsibilities completely just because they view their vacation as "private" time. This is true for low to mid level employees, but the more responsibilities you have, the more your actions can affect the company bottom line.