Of them all I prefer the orange background one (referred to as "Garish" by one member... which I can see from that perspective). I also very much like this one that looks very similar to the website logo. The white "Chess" and orange ".com" is a nice contrasty touch... without being Garish. 
Of the others that seem rather minimalist, I prefer the green checkerboard to the black and white one (The B/W variety is way to drab for my tastes). I also prefer the offset close-up pawn to the centered close-up. I think this pawn is too close to the viewer. I think this one is just right. Maybe add the logo in this one to it.
My rating:
I second this, not only the choice of simply the pawn with the "C" but also the explanation of the idea. 
If you had asked me to envision the iPhone's interface from memory, I wouldn't have remembered that there was a title underneath each icon. I simply look at the icon and my memory tells me what the thing is. (BTW, I tend to be a very visual learner, so that might be telling)
http://www.chess.com/photos/view_album/erik/app-icon-for-chesscom-app ?