It's wayyyy different in the US, i only skimmed but the concluding post for me was
http://xenforo.com/community/threads...-2#post-305332 in the US you specifically have to have "transfer of copyright" and you can only transfer copyright of an already created work, when someone signs up to your forum they haven't created a work so the US will ignore that agreement that they consented to prior to posting, however it does seem that you can have a pop up with the submission button for a post indicating that they agree to "transfer of copyright.....blah!" when submitting the post, that said you couldn't do it retrospectively for all previous posts.
Here in the UK (EU) copyright laws are very different and more defined.