The best electrical engineering advice I've ever gotten came in two parts one. First, when your circuit is behaving strangely, check the grounds. Second, only one point in your circuit is truly ground -- everything else is just close.
I just discovered, via Dangerous Prototypes, a great Maxim app note on the subject. For safety reasons, we generally want ground to be pretty close to our surroundings. But we still get to choose what it is and what its value is. Typically, the negative terminal of your power supply or battery is a good, safe place. But it's not the only choice, and freeing your mind acn lead you in some interesting directions.