FX cpus do that. Not just with a nice zalman or a noctua, but also with a stock AMD cooler. It is so common and I've seen so many threads on this, that actually I now consider this to be the normal behavior of top of the line AMD cpus. Bulldozers are worse. My room mates 8120 reports 19C with an ambient temp of around 28-30C.
I've not come across any accurate explanation on this and only some (near majority) and not all chips behave this erratically. No one seems to know why is there a negative offset or is it an issue with manufacturing tech. I'd say you are fine. Monitor your socket temps (CPUTIN), which is always higher than the core temp and just ensure it's a safe distance(actually it can be equal too) from the maximum specified operating temp of the cpu package (TCase).
Unless you are observing thermal throttling and performance hits, no need to get anxious.
good luck
-Satyam