Turns out that the location to place language files is wp-content/languages/bbpress/ and not anywhere inside your bbpress plugin folder.
It's an odd location for language files, though, considering that most plugins need to be self-contained and have a /langauges folder within the plugin folder. (e.g. within in /wp-content/plugins/bbpress/bbp-languages folder which already exists)
Also, the name of your file should be of the format bbpress-de_DE.mo not simply de_DE.mo
Finally, you should download the file from http://translate.wordpress.org/projects/bbpress/plugin/ not from http://svn.automattic.com/bbpress-i18n/
In order to download the file, click on the language you want, then scroll down to the bottom of the page, and hit Export.
At the very least, I wish bbpress.org included all this in their documentation. The existing documentation has nothing to do with how bbPress work now...all documentation is really for the discontinued standalone version. (by the way, the discontinuation of the standalone version should also be clearly stated on the download page)