Max Wideman's site has one of the best diagrams I've seen on estimation accuracy. He calls it the "Estimating Accuracy Trumpet."
The diagram is part of an online presentation on estimating. Once there, you can click on the arrows to navigate through the rest of the presentation.
This is part of his "Issacons" (Issues And Considerations) section, an indexed list of brief online presentations on just about every project management topic you can think of. From time to time, we'll feature interesting Issacons here, such as this one.
The Estimating Accuracy Trumpet