Yeah, mine do that too. It tends to happen when they are deep in REM sleep.
I think that they are dreaming of chasing rabbits. :-)

A cat will dream just like you.

   http://www.peteducation.com/index.cfm?cls=1

           Studies have shown that cats can enter into a deep sleep
like us, and have similar brain wave patterns as we do when we dream. So
if you see your cat asleep, whiskers twitching, and the eyes under the
closed lids rapidly moving to and fro, your cat is probably out on the
hunt in her dreams. Wish her 'Happy Mousing!'

The funniest thing to see is when they wake up suddenly and hiss.
Kitty nightmare!
--
Becks
http://www.toontalents.com/
~~~~~~~~~~~~~~~~~
Charles Montgomery Burns --
 "Oh, meltdown.  It's one of those annoying 'buzzwords.'
    We prefer to call it an unrequested fission surplus."
~.~.~.~.~.~.~.~.~.~.~.~: