Gonzo would not go NEAR the cats litter box, but my sister Chi used to jump up onto the shelf where the litterbox was kept to get into it I think little dogs have more of a tendency to do this... as far as I've heard.

I would suggest either putting it behind a babygate, putting the box up high where the cats can get to it but the pup cant, or getting a litterbox cover with a top entry (you can find these at Petsmart, Petco, etc). Eating kitty litter, especially clumping, can SERIOUSLY damage your dogs throat and tummy.. plus, theres tons of bacteria in there. And who likes cat poo breath? ewwww