My PC is always giving me mild electric shocks and there's rarely a time when peripherals don't spark when I plug them in! I'm always causing restarts and buggering up my graphics cards when plugging in my projector. I've noticed sparks plugging in my XL2 via firewire when both are on, but ONLY when using the DC coupler.
Using a battery you should get no earth differences, hence no sparking or shorts, plugging in the cam.
As I say, mine has sparked but caused no problems. Firewire is, by design, plug and play, so there should be no need to power off both machines before connection.
