IBM has its Social Computing Guidelines. Dave Evans, ClickZ's social media columnist, points to IBM as a good example for other companies to consider.
A quick search for "social media policies" turns up these guidelines from Intel. Best part, they are written in plain English.
Considers this example:
"Did you screw up? If you make a mistake, admit it. Be upfront and be quick with your correction. If you're posting to a blog, you may choose to modify an earlier post -- just make it clear that you have done so."