It’s still very much in its infancy, but xubuntuguide.org is starting to shape up. Nice colors, and while it’s not strict Mediawiki software, the (Tikiwiki-plus-Mediawiki) style is something I prefer in a wiki, personally.
A lot of the discussion and ideas for the guide are being bandied about on the xubuntu-devel mailing list (where I lurk, but don’t reply ;) ).
All this on the heels of this weekend’s wiki rampage. Sigh.