[_ck_]

Sorry I posted some incorrect/incomplete info. Until recently, it was only possible to have one database with both the wordpress tables and bbpress tables shared inside.

It’s now possible to force separate databases since 0.9

However I recommend against separate databases unless you have some kind of weird configuration that demands it. It’s just more overhead, more configuration, more to backup and worry about. Some hosts even limit the number of databases you can have (though tables inside are not restricted).