i'd have opted for edgewall's traq, which looks seriously nice (which fits with roundcube), but that's beside the point, 'cause it is one of those decisions where thomas has to have the last say :-)
as for adding to the docus: i'd be happy to share my experiences on a debian sarge postfix/courier-imap (using a virtualtable) with others, and a wiki would be quite the right place. if we can gather up enough sample-setups, it will be much easier for someone to reconstruct what went wrong (plus, we'll have a central focussing point for what _can_ go wrong).
Of course, it'd be a boon to the project, plus relieve some of the tech support queries that keep popping up on the dev- list that ppl have complained about. I've used MediaWiki at jobs, and it's very easy to setup. Keeping it open for all registered users would be the way to go (save for the front page) IMHO. I used to update the Hula Project wiki and it was a great resource (currently hard to navigate...)
So what do you say Thomas? http://wiki.roundcube.net/ ? Would you need someone else to setup the Wiki? I'm not a web designer, but find the default Media Wiki fine for starters.
P