One of the most common issues I find with organizations is that they confuse having a social media policy with adopting rules of engagement for how their stakeholders should engage in participatory media. While they do overlap (some examples of social media policy go into detail about how for an example an employee should behave in social networks) they are not the same. The difference in my opinion lies in scenario planning.
Defining rules of engagement for how your ambassadors should engage in public spaces means taking into account a variety of scenarios and planning multiple outcomes. If X happens, you should do Y. If Z happens, see plan A. Below is a flowchart I've constructed for community management to illustrate multiple scenarios and provide "rules" for how to manage them (click on image to go to Flickr source for full size viewing).
But flowcharts aside, rules of engagement for individuals and organizations are more than documents which lay out guidelines. A flowchart is simply a starting point and your corporate culture, leadership and individual discernment become critical when executing against the rules of engagement that have been suggested for you. In other words, once you have a plan in place, your organization will need to empower you to improvise accordingly. This is the art of engagement. Knowing the difference between policy and rules of engagement is like knowing the difference between a battle plan and actually going to battle. The plan is essential for a blueprint, but conditions on the battlefield change rapidly. And most importantly, you should put some thoughts into the rules of engagement for your organization's ambassadors if you actually plan on engaging vs. simply broadcasting.