Mike Cohn, of Mountain Goat Software, has a new agile book out that focuses upon planning and estimating. He has made some of it downloadable. In the Techniques For Estimating chapter he describes why estimates are usually way off the mark. I found the accuracy curve kind of interesting. The accuracy of the estimate increases as the effort put forth to come up with the estimate increases (I know... no sh&* Sherlock). But the unexpected part was that accuracy peaks and then declines with increased effort. I guess it is like over studying for a test kind of a thing. So the author came up with a fun way to increase the effort to the sweet spot that results in a more accurate estimate -- Planning Poker....