Ryan,
I'm a huge fan of GFCI protection. If it were up to me there'd be more GFI's, not less.
But they do nuisance trip. Whether the nuisance is actually a hazard, at least in my mind, isn't clear.
Winnie,
You said two things that make sense to me. I'm going to add a bit to one thought.
Capacitive coupling. If you had coupling at some point on a voltage divider (the resistance in the windings of a coil create a voltage divider) you would have unequal currents at the source.
The idea of flux imbalance at rather large currents. I actually keep a GFI circuit on my desk. It's a pretty nifty set up. The area where the current is sensed is neat and symetrical and it looks like, by virtue of the design, that stray flux wouldn't be a huge issue. But it could have an effect.
