Effect of Mining on Hardware
I have a 2012 Macbook Pro mining Litecoins (CPU&amp;GPU at a combined 80kHash/s) and a 2011 iMac (71MHash/s on its HD5670) mining BTC. They both run overnight and on weekends, I turn the mining software off over the weekday. Doing this for months now. 

So far, there were no overheating problems, but i have an extra cooling stand for the Macbook, which i would recommend.

Still, I expect the hardware to die sooner. The heat management in Apple machines is very delicate because of the case design, and I heard of people waking up to a dead iMac.