There have been a few follow-up questions on the spay tattoo post. To better show what I'm talking about, here is an old picture of Melly in which her belly is still shaved, thus allowing an excellent view of her tattoo (it's the dark dash on the lower portion of her stomach).
I did a bit of reading yesterday on the subject of spay tattoos, and it turns out that many, but not all, veterinarians that spay and neuter cats use tattoos to show that a cat has been "fixed." It's more common with organizations like animal shelters and rescues than with private veterinarians. Apparently the practice started with feral cats: vets would notch or clip an ear tip so that neutered cats were readily identifiable and would not be put through the trauma of being re-trapped. Some vets actually clip the ear of non-feral cats too (ack!), but tattooing the belly, the inside of an ear, or both is more common. Since not all spay/neuter operations result in visible scars, the idea is that a vet about to perform a surgery will see the tattoo and stop before opening the cat up.