[5Catz]

I went and did some reading up on other folk's experiences with their cats tongues hanging out thing, and most seem to be overweight cats who starting gaining significant adiposity at about age two or three. Also, some indicated that comaprred to the head size (which was often spoken of as "extra large") - the jaw/mouth size was small. Some also reported indications of a feline Down's Syndrome or some physical characteristics of it. (I read about twenty different webpages and had to deep-surf most of them find related "tongue" info) .
So most reported: fat and/or big boned cats with big heads but comparitively small jaw/mouths. A few also reported "divided or separated teeth" on the upper palate, or small or missing teeth; thus the tongue tending to "tip" or peep out. A good amount of the cats I read about were also Persian or cross and so maybe some of the characteristics of those cute squished-in faces passed to young could be a partial cause.
Just guess work on my part.
Thought some might find this interesting.

 

[Annissa]

that is interesting. I never thought about a correlation between size and a tongue poking out.

Sabby's only pokes out when he's upset.

 

[kungfukitty]

I've had many cat's over the years and I think they all had the tongue thing to some degree or another. Some have ben a little chunky but some have been average weight. None of their heads have looked overly large, either. They seem to do it when they're really relaxed and happy- it just kinda slips out. Regardless of the reason why I can't help but touch it. Call me weird.