Let me second PeterH's response. You can't rule out the effect of gravity unless you know what way the system is pointing. This is why most systems that use accelerometers also use a three axis gyro to sense orientation.
That said, gyros come with a lot of extra overhead that includes calibrating them to know what flat is when you start, and compensating for the fact that they will slowly drift off course. If you can find a cleaver way in your given application to sense what you want without knowing your orientation (The magnitude of the vector might be one place to start) that may be a simpler solution.