I had the same thing in my house, the upper floor of which was wired long after the house was constructed, and was an obvious case of "homeowner wiring". The upper floor was wired with an ungrounded "Edison circuit": that is, two 120V hots and a shared neutral.
This is a really bad idea for a lot of reasons, but here are just two:
- Though the voltage on the shared neutral is always zero, the current through a system is constant. Which means that in theory you could end up with a situation where there was 15 amps on each hot and 30 on the neutral. The circuit breaker is on the hots, not the neutral, so it will not trip for an overcurrent situation, even though the neutral might be getting dangerously warm.
If the circuit is wired in this way then it is wired incorrectly; in a correctly wired Edison circuit, the currents subtract rather than add. Every Edison circuit I've seen so far in a real house has been wired incorrectly. If you work out the sequence of events which led to this situation, often you find that the circuit was wired correctly when it was installed, and then a later modification caused it to become incorrect. Use caution when modifying existing circuits if you do not have a complete understanding of the whole system.
- A GFCI measures the difference in current between the hot and the neutral. I discovered that my upstairs was an Edison circuit when I installed a GFCI -- figuring that if I didn't have a safety ground at least I could have ground fault protection -- and it triggered at random. Clearly the neutral carrying the current for both hots means that it is often going to be unequal to the current on one of the hots.
Commenter Tester101 below notes that this only happens if the circuit is miswired; again, in my house, the circuit was miswired. It might be in yours too.
Fortunately for me the Edison circuit in question was wired up after construction of the house and the conduit it was in was easily accessed. I replaced it with a proper pair of circuits, each of which had a dedicated neutral.
Good luck! These can be a real pain to deal with.
If you're going to be rewiring stuff in the kitchen and bathroom anyways I encourage you to bring the whole thing up to code: dedicated GFCI circuits in the bathroom and kitchens, at least two in the kitchen, and dedicated circuits for high-initial-current inductive loads like dishwasher, refrigerator, microwave, disposal, and so on.