I thought perhaps that actually PNG can do up to 16 bits per color channel, meaning it supports 64 bit truecolor? Or maybe I am thinking of something else.
It rings a bell, but I think it might be an unofficial "branch" of the PNG format, rather than something officially recognised by the PNG Development Group.
Incidentally, I don't think Gav was confused nor indeed was he strictly wrong. There is a 256 color 1 bit alpha version of the PNG format, it's just not the only one.