"It's Gone All Pear-Shaped!"

[Mark A Mandel]

Someone asked on a newsgroup about the origin of the phrase "gone all
pear-shaped":
        >>>

I know *what* this phrase means, at least from context--an expression
for a plan gone awry.  (BBC America shows Ground Force, where I heard
Alan Titchmarsh utter it.)
        <<<

Followups have suggested
 1. the devolution of the male physique in middle age,
 2. what happens when a balloon begins to deflate (quoting Partridge),
and
 3. the discovery that the Earth is not only not spherical, it's not
cleanly spheroidal either

Any data or other reasonable ideas? (Note: If replying off-list [though
why would you?], remove the obvious spam-blocker from my edress.)

-- Mark A. Mandel
   Linguist at Large