Looks cool! I'm guessing the ZIF socket is too tall to be able to use shields with the board. Is that true? Otherwise, you might want to keep the pin location the same as the other boards.
Also, do you think people will be using this board to prototype with, and then removing the chip to use in a permanent version of the circuit? Or will it be more a tool for people looking to, say, burn the bootloader onto a bunch of chips. If the latter, you might consider making it without any of the USB circuitry, to keep the cost down.
This is just a question for my own education: how are you handling GND on the board? It looks like the whole PCB is a big ground plane (since none of the GND pins are connected to anything), but how can you tell that? What makes it so that the ground plane touches the GND pins but not other pins or traces?