Sorry for not explaining it better... the uShield Shield is a full sized shield that holds the voltage regulator and the additional i2c ports. The uShields needs to have just the i2c device and some additional components. Even the i2c bus adapter is a uShield, not a full sized shield.
So basically we were on the same track separately. That' a good sign.
Why wouldn't the bus adapter be on the main shield though? I have fairly limited experience with i2c, so maybe the answer is obvious.
Also, this is starting to sound a lot like http://www.phidgets.com/