Because agile is so much more than a process or methodology, but it redefines interactions, teams, and communication... it is hard to put into a perfect box and hand to someone.
So what do we do?
Mike Cottmeyer took a really good stab at this on the Agile Software Development blog. He takes the approach of pointing out what issues project managers face and then discusses how agile addresses them. Once you have their attention, then you can go into some of the deeper concepts behind agile.
Here is an excerpt from "How to Talk to Project Managers":
...ask them what happens after they communicate the project costs and delivery date of the project? I would bet money that their response will be somewhere along the lines of: we are given a date, we are given a budget, and then we start de-scoping the project.
...This is where you can explain that they are given the date and the budget because those were really the project constraints to begin with, not the requirements. We pretend the requirements are the constraint because, like most people, stakeholders hope that we will be able to get everything we want for what they are able to spend. Most of the time that is not the case.
...The reality is that by demanding ALL the scope, in the face of data that says otherwise, the business is making the implicit decision that their project will be late. You are generally not rewarded, or very popular with your stakeholders, if you bring this to their attention. Project managers are often incented to keep their heads down, do their best, and take their lumps when things are late.