Using the ST LSM9DS0 IMU or the InvenSense MPU-9150 both seem like a great choices for this type of shield. SparkFun has breakout boards for both and repackaging the oblong sticks into a Tinyduino shields would be trivial. Both chips support I2C, and the LSM9DS0 also supports SPI. The LSM9DS0 is pretty cheap as well- $11.61 in single chip and $5.85 in 3000 volume (Digikey).
In the case of the LSM9DS0, setting the other lines for interrupts, I2C/SPI select, Accel/Mag Address Select, and Gyroscope address select, would take up a HUGE amount of pins. Perhaps using a TI TLC59116 16-Channel Fm+ I2C-Bus Constant-Current LED Sink Driver ($3.23 and $1.25 in 1000 quantity [TI]) could be repurposed to be a pin selector using only the I2C bus. That way, only a few pins (2??) would be needed to use the LSM9DS0, which is always a good thing! Check thread
http://forum.tiny-circuits.com/index.php?topic=31.msg643#msg643 for further discussion on the TLC59116.