5870's have 11 different sensors.  I dunno about others.  They're all visible ont he i2c/smbus connection, the AMD GPU Clock tool in windows shows them all, I think GPU-Z does too, I haven't ever actualyl had a GPU based card in a Linux box, I'll know more about that next week (I work with servers.  Most of the machines I use only see a keyboard long enough to get serial console or their IPKVM working)