Well I'm not entirely sure a wiki is what I want. As a text writing environment I think wiki's are pretty poor but they still seem to be best game in town for attracting participation and that's mostly what I want.
I'm trying out DokuWiki which seems pretty simple and pretty nice. It supported ACL's out of the box and seems to have anti-spam measures available. I've seen too many wiki's descend into spam hell to consider any wiki that doesn't have a modicum of defences available. It also has a reasonable WYSIWYG editor, image upload, and some other nice features and, if I don't care for it being PHP, I've been able to grok the code when I had to look at it.
I started with the Ubuntu install of doku but couldn't seem to get it up to date (which meant horrible messages at the top of every page) so I just scrapped that and started again, downloading the latest release and installing it myself.
I'm using a rather ancient version of nginx which I, again, seem to have installed myself. I should probably switch back to the Ubuntu one (I think maybe I couldn't figure out how to install that when I originally started on Slicehost). This made setting up the clean url's harder but I got there eventually.
Now I have a working wiki and it's not looking too bad. Especially after adding a couple of the plugins to support things like inline notes.