I've looked at a number of solutions in this space for my company as well. None were EXACTLY what I wanted, but a few came close. A lot of them are coming out of the file sharing space - consumers sharing individual files with one another - which doesn't translate well to corporate file servers.
I recommend checking out Box.net - it has robust roles-based permissions and may be exactly what you're looking for. I ended up not using it because it didn't support some of the file types our team uses.
An interesting alternative is Egnyte, which also offers good role-based permissions, and supports cheap local storage as well.