I haven't experienced a transition from waterfall to Agile myself - I worked for companies using Prince2, waterfall approaches to managing projects, and then I got a job working for a company using Agile. But for most traditional testers, their first exposure to Agile will be when their company decides to adopt it.
What resources are there for testers in a company transitioning to using an Agile approach? What problems do they tend to face, and how do they overcome those problems?