Attended a estimation seminar yesterday morning. Some fluff, but being exposed to the theories and practicalities of Planning Poker was worth the time.
Used it immitietly in the afternoon for a project that we were still struggling to define the tasks needed. Very effective, in just getting a discussion going, grouping tasks and assigning estimation units to each task/story.
Well worth a try, even if it made us look like we were playing poker all afternoon, especially as we sat in a tiny glass cubicle at a client.
Agile estimation book by Mike Cohn