All right, so I'm currently leaning very heavily towards
Vanilla. It's a GPL project, fully funded by hosting and support services. The idea behind it is that the core software itself is very, very feature-light, and almost everything - quotes, who's online, you-name-it - is tacked on by plugins, many of which have been professionally code-reviewed and blessed. It's our best bet for getting something like a bespoke forum setup without writing the damn thing ourselves.
And, while I don't necessarily want to bring this up but I know somebody else will: It's the same software that the Penny Arcade forums use. Gotta figure, hell, if it's got the social options available to keep that group from trying to literally assfuck each other all the time, there must be some magic it. And to be honest, I like the idea that PA Inc. has probably been making its own contributions to the project for a while now.