Pete, thanks for putting me on your list.
The pre-reqs to become a PADI Cavern Instructor, as it was told to me by a PADI instructor, are you must be a PADI Instructor, certified as a Full Cave Diver from a recognized agency (NSSCDS, NACD, TDI, IANTD, etc), and have logged a total of 25 cave dives, some of which may be the ones you did during your training to become a full cave diver. If my information is wrong, I hope someone will correct me.
Every other agency I know requires that you be full cave with a minimum of 100 cave dives post full cave certification. Many require that you also intern with other cavern courses before you can teach the material.
I'm just stating facts, not making opinions.
BTW -- in 2000 and 2001 I was in the process of becoming a cavern/intro instructor with both the NACD and NSSCDS. In order to do that I had to intern with at least six classes at various levels with four different instructors before I would be allowed to attend an institute where I would be examined and evaluated by two different instructors. I made it about half-way before dropping out after the death of a friend (having interned with Tyler Moon, Lloyd Bailey and Steve Berman). Last year I started the process again (this time just focusing on TDI), and have interned with guys like Edd Sorenson, Jim Wyatt, Bill Oestreich, and several others (some of which are on Pete's list). I'm in the middle of my TDI full cave internships right now (just did one with the vice-president of the NACD last week).
In my opinion, cavern is a safety course. Anyone who has gone through a proper cavern course should understand and appreciate the limits and risks associated with breaking them. Sadly, not all cavern courses are created equal, I've seen guys from out of state break pretty much all of the rules while teaching a cavern course.