I've been looking for any sort of postgres support, and I see several forum posts mentioning a community project, but links to that seem to be 404s. Was this removed?

Yes it has been gone for quite a while actually :)

As an aside, the client who was developing the driver recently emailed me and told me he is no longer maintaining it, even for himself. He has found that a properly configured MySQL setup can perform just as well, so it's not worth the effort to keep updating his driver through our releases unnecessarily.

Okay. I'm trying to get IPB to auth against an existing system that uses a postgres table for auth. This isn't something I can change. My hope was to take what was written for the pgsql driver and use that as a basis for talking to our server (I'm fine with using Mysql for the rest of the forum storage). Does this sound like something that's reasonably doable?

It's doable of course, but without the driver available you're probably just as well off using pgsql_* functions directly in your custom code (or using another library - PDO or Zend DB maybe).