In this demo, Pat Hickey from Invensense does a demo on a turntable showing how much error you can expect from an IMU (in this case the MPU 3000 using the onboard Kalman filter) in a turn if you don't have a GPS or magnetometer to correct it. The answer is less than 10 degrees at .2G (12 inch turntable turning at 33rpm), which is a reasonable guess of what a plane would experience in a prolonged turn.
This is relevant because the forthcoming Invensense MPU 6000 sensor with 3-axis gyros and 3-axis acclerometers on a single chip has a great onboard sensor fusion processor, which is attractive to use. But it doesn't have an on-board magnetometer or a good way to input GPS data into the sensor fusion processing, which could correct the inertial errors from centrifugal forces.
So what if you go without that correction? This demo suggests that the consequences might not be too bad.
(demo filmed by me at the Wired offices)