I prefer the speakers not be underneath the laptop, but I understand that apple probably couldn't find a better place to put it.
I just know from using my wife's 10" netbook (with speakers underneath) that when I watch a video, the sound gets louder when I lift up the computer to move it. It's just weird to me. The one benefit to a netbook is amazing portability. I barely feel it in my bookbag. Now, the 15" pc laptop I'm using now is pretty heavy to me. I don't know what it weighs, but I should find out and compare it with the 15" mbp.