How much mhash/sec are you getting?
Well I'm kinda new to this so maybe my numbers are off. The Diablo miner says I'm getting 80,000-100,000 which I guess is 80-100 mhash/sec. My deepbit mining account says I'm averaging 35-45 though which I think is typically low but about where OSX normally is.
Then you are using a 6970M not a Radeon 6970.
It has next to no cooling; There's just a small pathetic fan on top of it, enclosed in the iMac. It will die from 24/7 mining.
The design is made assuming sprees of a few hours, after which the device will cool down.