ST is giving away their discovery kit board for free (click Register for your Free Kit button):
http://www.st.com/internet/evalboard/product/254044.jspIt has a Cortex M4 chip and comes with a magnetometer, accelerometer, and gyroscope. I have one in my possession, but I have not played around with it yet. It's not as small as the IMU boards from SparkFun but still small enough to mount on an HMD.