All late model Mac laptops come with either Intel HD graphics, or nVidia GeForce "M" solutions, neither of which are good for mining. It will cost you more in electricity to keep them running than they will return at current value, not to mention ruin your overpriced hardware via overheating. The late-2011 iMac model comes with a special slim version of the Radeon 6970; if you want to mine for BTC with Mac hardware, this is your best bet. Apple as a whole isn't very bitcoin friendly..