It get's the 3 basic pages from you bbPress install (front-page/forum, statistics and the search page) and puts them in your header. In the admin area, you can drag and drop these pages in the order you like (or hide them).
With version 1.1 you will be able te rename them aswell (so you can change Forums to bbBoard or something).
bbMenu 1.0 is compatible from bbPress version 0.8
Demo (no admin) can be found here: www.sourceskins.com/bbpress
To put this into a sidebar should be quite izi to do, there is a site link in the documentation file in the .zip with some explaining of how to change the menu's appearance.
Feature bbPortal releases will have a bbMenu widget you can drag into your sidebar....