Agile Managers
A typical problem in an organization "starting to be agile" is that some managers (typically middle managers) don't see where they fit into the new process. So in order to complete the transition successfully you need to have especially middle management in the plan. At a talk at Scrum Gathering 2008 it was mentioned that middle managers should be involved early and encouraged to participate in the process of identifying their new roles.
So what are these new roles? If we use Scrum as an example (because it is popular and has three well defined roles) it is obvious that managers typically don't are part of the team. They're manager and often cannot contribute to the sprint just like any other team member. Managers also typically dislikes being demoted to team members. They worked hard to be managers...
So many managers thinks Scrum master sounds like a new cool title for them. I think this is where many companies go wrong. The Scrum master is not a manager. It's a coach, facilitator and guardian of the process, the team and product owner. As such the Scrum master must be skilled and be really excited about the cause. Another warning. Even when the manager actually has the skills and desire a Scrum master needs it might still be a bad idea making him a Scrum master because of the risk that the team will hear him speak as a manager and not Scrum master hence "following orders" rather than "listening to advice".
So that leaves us with the product owner role. In my experience companies successfully adopting Scrum tend to make their former managers product owners. This way their old management backpack does not affect the team's evolution and also the managers are doing what they've always done; prioritizing and deciding what the team should do (but now in a new way).
Sometimes the manager is not skilled enough to be a product owner because someone else has more knowledge of customer requests and so on. Don't panic! In my experience people with good knowledge of what the customers really want often feel they have little or no time to spend with the team. So even though this is not an ideal scenario the former manager can step in as a product owner proxy when needed. You can also be assured that the team will come up with a list of impediments they cannot solve. So you as their former manager can come to their rescue help the team unblock some impediments.
And even if you're not the Scrum master nor the product owner you can still help the team by coaching them toward "better". For more tips on what managers can do read about daily stand-ups and impediment involvement.