Michael Stephens has written a post called Social Media Best Practices for Libraries. There is great content there, but it certainly does not spell out a suggested policy for a library. He does that in a separate post suggesting a policy for Anytown Public Library.
There is a great article in Public Libraries (November/December 2009, p. 23-25) by David Lee King and Michael Porter about dealing with comments. (I can't find the electronic version on the ALA/PLA web site.)
Jill Hurst-Wahl has a great series of posts on rules for using social networks. Here is #9.
Is this the end? I don't know. I think it will develop over time.