That's a great question!! I've wondered about that myself.
I took Organic Chem I and corresponding Lab as Pass/Fail. I took the class my first year of grad school and didn't want to have to worry about the grade in that class. I did take Organic 2 for a grade, however. I also had gotten AP credit for my English class requirements.
I, too, wonder what the adcoms thought about "Pass" vs. a grade. It was never brought up in any of my interviews, and I'm heading to Texas Tech in the fall. Apparently, it wasn't much of a factor for me, but I'd love to hear how it's affected others.