I recently read a great little article about agile planning by Martin Fowler. I find that one of the more difficult tasks to do early on in an Agile project is to get the team to as a whole, take responsibility for the amount of work taken into an iteration. Often, individuals will take on work that is role-specific (e.g. Dev work) without consulting the whole cross-functional team and the product owner.
I haven’t read it yet, but I suspect that Mike Cohn’s new book, Agile Estimating and Planning, might have some excellent ideas about all this.