most segways use a gyro instead of an accelerometer. From my point of view a gyro is mathematically much easier to handle for this inversed pendulum what a segway actually is (just integrate the gyro values over time, and you'll get the turned angle).
Gyro drift might be a problem for analog gyros, but accelerometer noise is also an issue.
I would recommend to purchase an i2c gyro - (this http://xgl.minfinity.com/
seems to be excellent! )
or try to use the integrated gyro of an android smartphone if you want to take these efforts (IIRC it directly returnes the tilt / inclination value) ...