Re: bbpress and memcache?
Glad to see what I am saying confirmed with independent realworld proof.
Pleased to be of service. 
BTW, just how big is your site that you require multiple db servers?
I don’t know of a single WordPress+bbPress install, other than Automattic’s that are using multiple db servers.
(unless of course you are running other apps that have heavy db use or maybe for backup replication)
It’s more for redundancy than anything else, although before we got opcode caching in and had optimised some plugins involved we were catching traffic surges when our client tweeted new posts that were making us very very glad we had that much capacity.
