Seems like with network filesharing you'd be happy with Google Apps. So what's keeping you from just buying filesharing from another vendor?
I really hope, that there were a good alternative for MS setup. Currently I feel that only a network file sharing, centralized directory for users (AD), and printer sharing are about the only things a little bit problematic without using MS stack. It's shame that so few file sharing / collaboration services integrate with Google authentication.
I was thinking about some sort of auth-proxy for authenticating against google apps, and in case google apps doesnt respond it would use whatever happened last time, as a cache. And that proxy could be used for PAM, network login, filesharing etc. But its not in the scope for this project to develop that.