I added a bbpress subdirectory to my theme and got a blank screen. I was using a “silence is golden” index.php file in the subdir and removing that fixed it. I guess the subdir acts as a subtheme or something (though it has no style.css file).
Interesting thing is I could also fix it by dropping in the files from plugins/bbpress/templates/default/extras to my main theme directory, following the instructions here: `http://codex.bbpress.org/theme-compatibility/
Posting this in case it helps anyone else.