Cancer-sniffing animals?

I was reminded by this post of a show I saw a few years ago that reported that some animals can determine when, and sometimes where a person is ailing. Through a variety of tests–that I can no longer recall, of course–researchers determined that some animals can pinpoint illnesses such as cancer, asthis story suggests.

Has anyone had any such experiences? Or do they have any links on the subject they’d like to share?

I have heard anecdotal evidence and I think there was a New Scientist article about the subject, that there have been cases of pet dogs repeatedly sniffing at and worrying at a particular mole or spot on their owner’s skin. Owner goes to the doctor to have it examined, and lo and behold, the mole or spot turns out to be cancerous.
I think there are more things in Heaven and Earth, Horatio…

The problem with anecdotal evidence is, we don’t see or hear about the negatives.

Case 1: The dog repeatedly sniffs at a mole or other skin lesion, person visits doctor, doctor determines it’s a harmless freckle.

Case 2: The dog completely ignores a skin defect, which later turns out to be deadly melanoma, which metastasizes; person dies, dog doesn’t give a dam.