See http://thelackthereof.org/projects/perl/oddmuse_pingback/ for source (and darcs repo).
I would appreciate it if people could link to this page in a pingback-enabled post of some kind so that I can make sure it works. The only way I've tested it is with this client, which has worked swimingly.
This is the first step. Second would be to make oddmuse work as a pingback client too. The trick there is to make sure we aren't over-pinging links and I'm not sure how that should be done.