Can you feed a subpanel with a 2 pole GFI where the subpanel provides 1 and 2 pole loads? Doesn't the GFI compare the 2 hot wires and trip when they are not equal? So wouldn't a 120 volt load trip the breaker because that would cause an imbalance in the 2 feeder wires? Or am I all wrong about this. Because what if the 2 pole GFI fed a piece of equipment that itself consisted of 120v and 240v loads (like a spa)? I'm troubleshooting a job where a 2 pole 50 amp breaker is feeding a small sub panel for pool equipment, consisting of a 220v pump, 220v heater, and a couple of WP 120v outlets. The owner added 3 ceiling fans under a deck and wired to one of the outlets. They worked for a while, then the GFI breaker tripped. I went there and checked the 3 CFL's, found no problem and they all worked without tripping the breaker.
Owner called later and said breaker tripped again when CFL was turned on. I'm thinking I need to feed the subpanel with a standard breaker and feed the pool equipment with individual GFI breakers. Anybody got any ideas?

I bet you have a ground fault. Those are typically sold as 120/240v devices and should handle both loads. I am assuming there is a neutral lead coming from the GFCI to the main panel bus and the neutral from the feeder is going to the GFCI. It will sum up all the current in and out, detecting a mismatch.

gfretwell: If it was a ground fault, I would think it would be present all the time. This might run for 3 hours before tripping. And the owner said sometimes it trips when the CFL's aren't on. Oh, and yes, the neutrals are connected properly.
I was leaning towards the GFI breaker going bad, but I'd hate to spend $150 for a new one and still have the problem.
Very difficult to troubleshoot when most of the time it works.

gretwell: So you think that the 2 pole GFI should not trip if, say, L1 has a 10 amp load, L2 has a 12 amp load and the neutral has a 2 amp load? I'll buy that, I really wasn't sure.

Yes, look at the write up on one of the breaker web sites., All 3 leads go through the current transformer and get summed.
As for what it takes to trip a GFCI, when the fault current is in that 4-5 MA range it is just waiting fir any little deviation to push it over the edge. You might be able to see something going from each leg to ground with an ohm meter but it is usually easier to just start eliminating loads and see if one is the culprit. I have worked on circuits that split out and went several ways. It was really hard to find the bad leg but I usually found one. It could be as simple as ants in a motor or water in a box.