Nice concept, but consider replacing the sockets used on the ISP board with zif sockets. The normal sockets your using are not designed for repeated insertions/removals and will quickly wear out.
Here's something I cooked up for my own use:
It's just the DIP socket broken out (Up?) to the shield and the 2nd socket is configured to burn the bootloader. Without the 2nd chip in place it operates normally and once projects are nailed down the chip can be removed for standalone apps. and another bootloaded chip can be installed for the next go round.
Pull both chips and it's a programmer:
(The sockets have to be empty for loading sketches. Unlike the pic...)
I'm planning on breaking out the pins needed for programming for use with a single cable instead of the jumpers.
But, yea, the ZIF sockets would drive up the cost.