It's due to the way ATI and nVidia designed their chips. In ATI, there is thousands upon thousands of miniature cores each capable of calculating simple problems, compared with nVidia's minimal amount of more powerful cores. Bitcoin mining is essentially doing simple calculations over and over again, and thus ATI's are better in this regard. An ASIC for bitcoin mining is an incredibly effective hardware for mining Bitcoins, purportedly running at over 60 GH per machine (your nVidia runs like 6-10 MH I'd wager, or like a couple of magnitudes slower). It's almost released by multiple manufacturers, although there's still no confirmed ASIC in the wild.
Earn a couple pennies this month vs a non zero chance of destroying a laptop. Hmm...
The worse thing is it dying after your warranty's over. Macs are really expensive to repair.
(edited on request to remove some stuff)