Re: bbpress and memcache?
Would be awesome to get your feedback on the new bbPress plugin for WordPress once it’s released. Since it uses custom post types it should play nicer with setups like yours.
Regarding your question, I don’t have an answer for you right away but should be able to get one soon. We use bbPress on all of the .org sites with a similar server setup, and someone with more experience on the guts of them should be able to chime in.