I like the idea as it solves a problem, however it introduces a new problem too IMHO. Shields are relative "BIG", sometimes as width as some breadboards, so this design could cover all the holes of the breadboard, or at least a substantial part. A better design needs to be able to connect to a breadboard, without covering it. So here my thoughts:
* Imagine a PCB in which you can plug a shield (you made that working allready). However the PCB does not plug into the breadboard but has header pins so I can connect it with standard female/male cables to a breadboard or to an Arduino or to some other thingy. That gives the freedom searched for I think. The cable can be some flatcable or so.
[in fact I used a screwshield - http://www.sparkfun.com/products/9729
- this way.
* The PCB should have 4 mounting holes so it can be used in permanent setting
* The PCB should have a LED to show that the +5V is applied to it.
Its funny that this discussion should come up, and in fact that these very details would be in your suggestions, as, as soon as this weekend I was preparing to release just such a board.
It is called the CideKick Breadboard Shield Stack Extender or just CideKick for short. The first small batch sold out locally before I could even post about them online.
The red jumper removes the pin 13 led circuit( the red led )
Power is noted on board by the green led
The breadboard rail is powered by two sets of 5v pins attaching the whole assembly securely to the stack.
Positive rail is 5v Negative rail is ground
The reset button is brought out to the CideKick for easy access for shields that don't supply one
The 20 pin inline female header is 13-0 digital pins and 0-5 analog pins from left to right
In the final picture the red wire is in the pin 13 digital, green is in pin 0 digital and yellow is in pin 5 analog.
The 6 pin center header(between the pin 13 led and power led)
are aref, 3.3v, gnd, reset, 5v, VIN
The odd shape ( the hook looking part in the upper right) is to accommodate the shield stack screw hole
Since I began with the Arduino there has sort of been a separation between my breadboard projects and projects that use shields, mainly because alot of shields don't supply the stackable headers or I dint want to always solder up a protoshield for each little change in a breadboard circuit, it completely defeated the purpose of the solderless breadboard Arduino combination many times.
You see this especially in some gps shields, some lcd shields and even things like the danger shield making it difficult to stack and run wires.
That is what drove my designs for the new Arduino Breadboard Tools, of which the CideKick is one, along with the WingThing, which addresses the very problem the OP posted about, shield incompatibility with insertables such as the boarduino and the ardweeeny, and the CINR which is a brand new format Arduino clone specifically for breadboard experimentation.
Sorry for the thread jack, but I figured this info was on point and relevant, as I said I will be releasing the full information on the Arduino Breadboard Tools this weekend, feel free to PM me for further info or any direct questions, in the mean time here is a quick peek at the CideKick.cidekick01 cidekick02cidekick03cidekick04cidekick05