A shield also wouldn't allow certain components to be shared, like the resonator - although, two 168's as a shield would.
As an IO shield, I don't know that you'd need a resonator, the AtMega should be OK at 1/2 speed on it's internal RC clock; otherwise, they're a matter of a dime or so.
But I'm not sure how you'd program the chips if it was a shield, I think you'd still need the mini-USB adapter - if anyone thinks up a way that this can be a shield in the next 24 hours, then let me know, we've got a hundred pcb's on order that won't go into production for another day or so.
If you did it right, the shield would only need to be programmed once, with generic I2C code, and never again. This could either be done by programming the micro in another board (a ZIFDuino?) and transferring it to the shield, adding an ICSP header and using a programmer, or adding a TTL-serial header and using something like the Arduino Mini USB Adapter or the FT232 cable.
I have a fair portion of the slave I2C code written, I'll see if I can write the master library and design a shield proof-of-concept in the morning.