Cat's lash their tails when over excited. I'm using over-excited to refer to any strong emotion, good or bad. though the cause of tail - wagging is usually anger, we have one cat who gets overstimulated just by being petted and shows it by wagging his tail. It's something he likes, but he can only take so much. The faster the wag, the more excited the cat is getting. A wagging tail can also indicate intense concentration on something, like a mouse in the house.
Dr. Seuss~DLH (brother's cat)~June 2007-
Misty~DSH (my cat & Mooby's mom)-?-
MooBoots(Mooby) ~ DMH(Mom's cat)~July 21, 2008-
Sunshine~ Golden retriever X white german sheperd (Dad's dog)~October 24, 2008-