Funny, only one of the dogs I've had so far did that on a regular basis! This is the only photo I have of her doing it, and it's a bit blurry because it was with a hand-held camera in dim light conditions, and no flash.
The paw curl may help with cooling since they "sweat" through the paw pads as well as pant to cool down. I also know a gal with a dobie that curled a paw when doing a down. She liked that since when he uncurled his paw it signaled that he was about to get up.