Re: bbpress and memcache?
Interesting, my opinion would actually be the opposite.
bbPress is probably faster in a shared environment than the others (if tuned).
SMF and Vbulletin already take advantage of shared memory like eaccelerator/apc/xcache out of the box and have different levels of caching. Vbulletin is very bulky though.
The magic to bbPress is not performance, it has an “acceptable” rating in performance for 1.0, but what you can do with it and how quickly. You can customize bbPress like no other forum I’ve seen with quite a bit of ease, not just the look/theme but the actual functionality.