The Wii nunchuck has 3 axis accelerometers. It talks thru an I2 interface, and the whole unit can be had for $18. The Wii MotionPlus has a two axis rate sensor (tuning fork) and is reported to be very accurate by gamers for another $18. Get two for 3+ axes.
Seems to me that this mass produced system is a lot cheaper than a 6 axis OEM setup.
Add a two axis compass from honeywell for a fast heading sensor, and you can do dead-reckoning without a GPS.