Well, if it is a bug, it's not just in 1.2.3 - i've just installed 1.2.4 (as i happened to have the sources for that version kicking around here) and iax2 show registry doesn't work properly with it either.
As with 1.2.3, when i do that command, i get the headers (Host, Username, Perceived, Refresh, State) but nothing else.
Any ideas?