OK I just measured the power through my iMac with a 6750 chip in it.

iMac not mining:              72 watts.
iMac running DiabloMiner: 88 watts.

So turning mining on adds 16 watts!  Whereas mining on my 5850 adds about 150 watts.

The iMac is not using much power for the graphics card!

The the electricity to keep the iMac on all day mining is only $0.34.  Whereas the 44 MHash/second earns you $0.85.

So the iMac is still profitable as a miner.

Sam