Sorry, wish I had more time, and I'd make a drawing. First, to be honest, this is just something I was kind of hoping existed, and someone could point me to where to buy it. I doubt its worth somebody working up a custom design for. But, since we're discussing it, I'll try to describe better what I had in mind.
The shield would be very basic - essentially a pass-through, but each of the pins would be routed to one (or maybe 2-3, total) ribbon connectors (or similar multi-conductor cabling). (Ideally, the reset button, on-board LEDs and ICSP would be replicated on the shield, as well...)
The ribbon cable(s) (connected to the board through the ribbon connector(s)) would then have a small DIP-spaced PCB on the other end - ideally in two rows of pins that can span the center divide of a breadboard, which could be inserted into the breadboard, thus providing access on the PCB to all of the Arduino pins. This allows you to get all of the pins to the breadboard by simply plugging in the small DIP-spaced PCBs, vs running a rats nest of jumper wires from the Arduino to the PCB.
This would not include a PCB itself - the idea is, you could plug the little adaptor gizmos into any breadboard of your choice - this simply provides the plumbing to get the signals from the Arduino to a breadboard.
On a smaller Arduino, the little DIP-spaced PCB (lets call it an 'adaptor') - the adaptor - could really just be a single item - an Uno, for instance, would just be a little 2x16 (or thereabouts) adaptor (ie, 2 rows of 16). The PCB could even be labeled with the pin names, so it'd be just like having the Uno on the breadboard.
For something like a Mega, you're now talking something along the lines of a 2x50 adaptor, which will eat up most of a typical breadboard just on its own (ie, no space for other components on the breadboard 
So, maybe it would be broken out into 2-3 adaptors - one for power/reset/refs - one for analog/pwm - one for digital. These could then be plugged in as needed or not, or even plugged into separate PCBs, if needed. Again, the adaptors would be labeled with pins, so its just like having the Mega on the breadboard - no need for the rats nest between the Mega and the breadboard.
The closes thing I could find like this is the Adafruit 'Patch Shield' (Patch shield for Arduino [v5.01] : ID 256 : $19.50 : Adafruit Industries, Unique & fun DIY electronics and kits), but I'd dispense with the 'patching' part, and just route all the pins through cabling to adaptors on the other end, each with its individual function labeled. Heck, even a super-duper patch shield would work - it'd just be great to have the pins labeled on the adaptor end, and I personally see no need for the 'patching' part - just route them all. Breadboard space is cheap.
In reality, eating up breadboard real estate is much less of a concern than avoiding the huge mess of wires between the Arduino and the breadboard. Breadboards are cheap enough that you can easily buy more real estate, but you if have one odd jumper wire come loose between your Arduino and your breadboard, you're in for a world of pain before you first identify that as the problem, and second get the errant jumper wire(s) seated back in the right place. This shield/cable/adaptor approach would make that a problem of the past. (Maybe I'm just a klutz - combined w/ the fact that most of my projects involve interaction with real-world things, in places where things get moved around, shaken, etc during development and testing - but something solid like this would make my life much easier!)
Dos that better clarify what I was describing poorly?
