I, too, was hoping I could use iChat or iChatAV with H.323. It's a widely-used standard for videoconferencing in distance education. I recently agreed to do something for a colleague's class at another university (Purdue), and I asked the I.T. help there about using an iSight on my Mac. He said H.323 didn't work with iSight or iChat. I checked out Kirk's reference to the "open323" effort, but I couldn't find anything that related to Mac OS. I'm wondering why Kirk referred us to this site? Did I miss something? It looks like a wonderful thing, but of no help to Mac users, unless I missed something. Also, I couldn't find any mention of "323" or "Polycom" (commonly used H.323 equipment for classrooms) on Apple's iSight and iChat help pages. So, there seems to be nothing on the openSource front and nothing from Apple for people that need to communicate using H.323 because the institution they are working with uses it (not our own ISP).
I vaguely recollect that H.323 is a proprietary Intel format. I wonder if Apple will be able to work with it soon. I hope so, because this is a major gap, with worldwide marketing impact for Apple, not just U.K. or U.S.