Inertial nagivation requires both accelerometers *and* gyroscopes. It's quite easy to find packages which include both instruments inside a single physical housing. The gyroscopes can measure angular acceleration – integrating this twice (if done with proper filtering) is sufficient to determine orientation in the absence of a gravitational field, so that software knows which direction the accelerometer readings correspond to.
If you are interested in knowing more about this there are some excellent references at the NGW suggested reading page in the wiki – try "an introduction to inertial navigation".
Btw: I've moved this to the nagivation subforum. The convention seems to be that "navigation" is figuring out where you are, and "guidance" is getting yourself somewhere you want to be. In our context, navigation is accelerometers, gyroscopes, radio doppler measurements, etc., and guidance is making orientation/velocity adjustments with RCS to keep ourselves on track. If people feel like this isn't the appropriate way to use these terms I am happy to discuss that, but this usage seems most in keeping with what I read around the web (I'm certainly not an expert on these issues).
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