Good to know, but then again there are publications that accept previously published material. And even though most publications don't state it I believe the first rights rule applies to most of them. Which means after a certain amount of time (usually 1-3 months) you can do whatever you want with your writing.

Personally, I don't do it very often, but there have been a few times in which I've quit writing for some publications and in those cases I pulled all of my stuff off there and either published it on my own page or submitted it to publications which accept previously published material.