So far, the information I've seen says that it is easier for mice to get cancer than for dogs or humans, so the technology is under investigation, but there isn't a very serious risk being suggested. Of course, if it's your dog that gets cancer that isn't very comforting, but in the general scheme of things, it's likelier for a dog to be lost or stolen than to get a tumor induced by a chip. With Sioux, my long-haired beauty, a tattoo would be lost forever under all that fur. And, to tell you the truth, my hound had ear tattoos, and neither the ACO that found him, nor my vet's office, ever noticed them. They turned out to be nothing more than litter numbers, and not traceable, but still...
So, for now, I don't regret having chipped the dogs, and I would not put them through surgery to try to remove the chips, although I would certainly think twice about it in the future. I'm not sure when this correlation was first noticed, but it certainly hasn't made me feel any better about either scientists or the government doing their job to see that new technology is as safe as they first think it is.