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Always, but it's never on my lap per se. When I sit on the couch (which is how I use my MBP 99.44% of the time) I always keep at least one foot on the coffee table, causing my one leg to be elevated, suspending the notebook above my baby baster. Thus, no heat on my groin and no risk of infertility.
 
What I don't get is that some of you guys are fearful of infertility, when I am sure you don't plant the mbp right on your crotch area. Even a 5 foot tall person like me has ample room on my lap to not even go in that area. Which makes me think you guys are are just joking...can't be serious haha.
 
I have used my MacBook Pro on my lap quite a bit, especially when traveling on the bus for hours at a time, and last weekend it was on my lap a lot as I typed up some papers and a novel that I have been working on. The new unibody MacBook Pro doesn't get anywhere near as warm as the previous generations.
 
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