If you're sticking your fingers behind the main panel then I sure hope you have a reference like page 91 of "Wiring A House"
The neutral wire connects to one of the neutral buses on either side of the panel. There should be a tie bar connecting the neutral buses at their bottoms so it shouldn't matter which neutral bus the neutral wire connects to. If you don't have a tie bar between the neutral bus then you need to convert your subpanel to a main panel by adding that tie bar.
The neutral is grounded at the utility transformer and at the main panel. The panel itself has a grounding wire from the neutral bus to a no-foolin' main house grounded earth connection (which also connects to the house's cold-water pipe). The panel also has a "panel bond" screw from the neutral bus to the panel designed to make sure that the panel case is grounded to the neutral and doesn't somehow develop a voltage.
Page 8 shows how it works. A 120V load takes current from the hot leg's wire to the load and back through the neutral wire-- in other words the current is flowing directly from the transformer to the panel and then out through the load and back to a grounded neutral. (The electrons complete the circuit by running from the house's earth ground back out to the transformer ground.) A 240V load uses each of the 120V lines (whose net difference is 240V due to their opposite polarity from a center-tap transformer) and doesn't include the grounded neutral at all.
So the grounded neutral has current flowing through it (which can be checked with a clamp-on ammeter), but it has zero potential with respect to ground (and hopefully also with respect to you, which can be checked with a voltmeter). You're OK as long as you don't become a more attractive grounding path than the 120V neutral wire or get into parallel with a 240V load.
It's pretty hard to be an attractive 120V ground unless you're soaking wet... GFCI breakers (& GFCI receptacles) are designed to detect the small leak-off current that has a couple microseconds to try to kill you before the GFCI trips. They're pretty cool little pieces of equipment.
If you happen to ground one of the live feeds to a 240V load then you are well and truly screwed for a few more milliseconds-- after that it's EMS's and probably your survivor's problem.
Submarine electrical buses are ungrounded on the theory that it's a more reliable way to get power to the equipment without having to worry about maintaining a grounded neutral return. It saves a lot of wiring on 450V AC and 120V AC feeds, too. (Some local compartment receptacles are grounded separately.) The ungrounded buses are tested hourly by watchstanders to make sure that a ground fault isn't developing, but sometimes the ground faults come looking for you and find you before it's time for another set of logs. However NAVSEA decided that equipment survivability was more important than the occasional fireball.
I hope this satisfies your curiosity without giving you the feeling that you're capable of wiring a panel. If you're planning to tackle some home improvment then do yourself a favor-- seek professional help or go read books from your local library or from Home Depot before trying this at home. I've seen too many guys get killed (or worse, injured) to try to figure this out on my own.