The people from turtleweb build a nice solution (free at the moment) which can be used to sync a shared Lotus Notes Contact DB with your BlackBerry devices. The basis of the solution is a fairly straight-forward 'persistent store & push listener' model. The idea is that the Notes database can push down contacts data (or other elements when you change the design), which is then stored locally on the BlackBerry. The BlackBerry app allows the user to search for contacts and to make calls and send emails directly from within the app.
More information can be found here.