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Adding a notice that cannot be avoided is merely another way to look like a huge corporate tool, and the site is already complex enough for someone brand new to it. Imagine if you went into a user group, and during the open question period you stood up, but was interrupted by some official person in the front speaking in fluent lawyer explaining your rights and recourses. Just because other websites do it doesn't mean we need to.
It will be a sad day when the lawyers finally convince the heads of SOIS that we must have a notice or click through agreement prior to someone posting a question.
Speaking from the other end - if a user is truly worried about who "owns" their content, they already know where to look and how to find out what a website will do with it. It's plainly obvious that it's going to be on public display, and those that care will readily discover, as it's on every page, that their contributions are going to be licensed under cc-wiki, just like wikipedia and so many other websites.
I don't think we would be doing anything other than confusing people and causing more trouble if we were to force people to read such a message prior to posting.
*As far as I'm concerned, a click through agreement includes simple notices, as they would indicate that continued use of the site falls under certain terms and conditions. In other words, if you click submit for your post, it's still a click through agreement.