Take a look at lurker. Have been using it for quite a while and it's pretty good.
Thanks Meng -- I hadn't seen that, but looking at it I'm quite impressed. The C++ underpinnings scare me a little -- C++ web applications seem weird -- but I expect the XSL layer will give me the flexibility I need. And the actual featureset looks great, so I think we'll end up using that. (I still like Xapian/Omega, so I may look into using that for website searches)