My 7 yr old male cat has the same "tummy flap" - it reminds me of how a fat
person looks when he/she gets skinny and has loose skin. He's never been
fat, but he developed this feature. I'd also like to know what it is caused
by. Does he need a tummy tuck?!! ;)

My cat has the flap, so I have checked this out.  It's not a problem
or an indication of obesity.  Abyssinian cats have the flap.  So do
American keudas, a phenotype that has since morphed into a breed with
registered stock (which is somewhat absurd, as Kitten Evaluation Under
Direct Assessment was a study to define traits found in the most
well-adapted barn cats).

Here's a picture of an Abyssinian show cat:

http://www.serve.com/BatonRouge/history.htm

Charlie

My Espy has a flap. He is by no means overweight and never has been,
but just has that skin flap.

That's pretty common in neutered males and spayed females- I've even heard
of it referred to as a "spay sway".

Three of my four kittens had this,  up until about 1 1/2 years old.
They are now two years old,  and the baby fat is almost gone
completely.

At about 8 to 11 months of age,  the flaps looked quite alarming,
if I didn't know any better.  The baby fat doesn't all burn off right
away,  after they are almost full grown (spinal length-wise),  so it
hangs down as they trim up and become more muscular.  It should burn
off in later stages of maturity,  between one and three years of age
(up to 5 years for ragdolls).   They may also continue to "fill out"
and become heavier,  though not apparently "larger",  as they gain
muscle mass.  This is normal.