Since you can assign functions to your mouse buttons in the Keyboard & Mouse pane and the Dashboard & Exposé pane, you may inadvertently override a function if you give the same button a new function in the Dashboard & Exposé pane. If the button is enabled in the Mouse tab of the Keyboard & Mouse pane (not set to Off), it will use the most recent setting you assigned to it, but the other preference pane may not always show the newest function.
In other words, if a Mighty Mouse button doesn't behave how you expect it to, go to the Mouse tab in the Keyboard & Mouse pane and reassign the function to the mouse button.
This document will be updated as more information becomes available.