If you haven't heard about it, Scrum is the most widely adopted development planning method in use today. (Well, actually, "hybrid" is the most commonly used method; that is, a hybrid of XP, Scrum, waterfall, and others.) Usually about half of the people attending our Requirements That Work seminar are adopting Scrum. Requirements That Work focuses primarily on requirements and the artifacts of planning, product managers are also interested in the mechanics of agile development.
If you want to know more about Scrum, there are many books and websites available with information--often too much information for what a product manager needs to know. This video does a very good job of introducing Scrum in a very short ten minutes.
Take a few minute to learn more. You may find that these techniques can be used in your product planning regardless of whether your company has adopted Scrum or not.