Health Tip: Clumping Cat Litter - Really Bad for Dogs

http://www.petplace.com/dogs/dog-ate-clumping-cat-litter-vets-advice-on-dog-that-ate-clumping-litter/page1.aspx

http://virtuavet.wordpress.com/2010/09/15/clumping-cat-litter-can-kill-a-dog/

Basic information - dogs who eat clumping cat litter can become obstructed.

From the link, signs of obstruction can include:
If your dog is vomiting, reluctant to eat or has a loss of appetite, acting lethargic, or straining to defecate

Some more info on cat litter: http://cats.about.com/cs/litterbox/a/clumpingclay.htm