Thanks to everyone who contacted me with wiki suggestions.
After looking at a lot of wikis, and finding none that really met my requirements (svnwiki and zwiki probably came closest), I decided to write my own. Currently it's some 300 lines of code that implements everything I need in a wiki for my project right now.
A few minor features are lacking (online page editing, search, RecentChanges, themeability). Actually, I've turned the traditional wiki concept inside out, and instead of a big engine that stores pages, history, converts a custom markup to html, etc, etc, my wiki just runs on a set of files, formats them to html using a standard formatter like markdown, handles WikiLinks, and writes out static html for the web server. It's a wiki compiler.
To let other people edit the wiki, just set up a subversion (etc) repository, let people commit to it, and run the compiler in a post-commit hook. Of course, it takes care to only rebuild pages when something has changed that makes rebuilding necessary, so a typical commit will take less than a second to compile and go live.
I'll probably add all the missing features as I need them, and might eventually release it. BTW, I'm calling it "ikiwiki".