AFAIK, the CESA is still an OW requirement. Who doesn't teach it?
I'm not a big fan of people using it in place of dive-planning and gas management, but as a last-ditch alternative it's better than nothing.
BSAC does not teach it.
There is a SB member I will not name but who should be familiar to most who must be on vacation or something. I would normally expect him to be all over this thread about how CESA is evil and should not be taught at all.
After one of his anyone-who does-this-will-surely-panic-and-die posts, I personally contacted BSAC for an explanation. I was told that BSAC does not teach CESA because it focuses instead on checking gauges and having redundant air sources available. I followed that with a statement just like Web Monkey's above--OK, so what if the unthinkable happens and you do find yourself in an OOA situation with no other air source around?
The reply was that you would do a CESA.
The implied attitude in that response was that a CESA is so easy (just blow and go) that there is no need to practice it.