To add to what the previous poster said, at the current price of Bitcoins it is still marginally profitable to mine on a Mac if you have a relatively recent AMD graphics card that has decent hashing power. See
https://en.bitcoin.it/wiki/Mining_hardware_comparison#Card_Comparison_Spreadsheet to find out what works best.
Generally, Bitcoin mining is moving to specialized hardware that will use less electricity for a lot more hashing power. Since Bitcoin is designed to only release a certain number of coins over a defined period of time, as a result the difficulty to find a block is rising and soon commodity hardware will no longer be able to mine at a profit, unless of course the price of Bitcoins goes up dramatically.