I'm going to go ahead and give this topic a little nudge in hopes that someone can shed some light. I've also found a couple of posts like this:
http://forum.pfsense.org/index.php/topic,29152.msg151679.html#msg151679
where someone was trying to use AH, and the workaround they came up with was to use ESP.
I'd really like to use AH, as in the eventual implementation one end of the tunnel will be a low powered device that I'd prefer to not saddle with a bunch of encryption, and in this application confidentiality is not as important as authentication and integrity.