Basically I'd like to know if someone who posts contents on my widget site can come back later to say he wants it taken down again. Is there a type of disclaimer that would allow me to constantly keep user contributions online, or is any type of permission revokeable if the content hasn't been paid for?
I'm also thinking about stuff that has been built upon. Wanting a brick back that someone has lying on his desk is one thing - wanting a brick that is incorporated in the wall behind the desk is another thing (possibly). Thanks!
You can certainly ask people to sign over copyright to you - but they are unlikely to do that, unless you are paying them.
If you ARE paying them, then your site should make it clear that you own what you've paid for (if it is not made clear before they submit, then you DON'T get to own it!
A better way is probably to say that they are giving you a "license in perpetuity" - and make that user friendly by emphasising that they do get to keep the copyright (but they can't stop you using it later!).
The way you phrase your agreements is very important, and (of course!), I can't help you there - but there's plenty of info on copyright and licenses out there.
Don't social networking sometimes tell you that they own the copyright of everything you put on there? I seem to recall some angry discussion about that a while back involving at least one of the major ones.