We have just made the first transatlantic NY/Southampton crossing of 2008 and judging from the announcements that we heard it seems that you can leave whenever your transport arrives. Passengers were certainly being called at 8am if not earlier.
If I were you I'd double check with Cunard's HO in Southampton.
The QM2 is a 'ship' not a 'boat'.
Hi - passengers dissembark according to their cabin / deck so its difficult to judge. If you are willing to carry or wheel your own bags off and through to customs you can leave right away othewise saft to give it approx 2 hours. I would go for 9am. Hope this helps unlike your other reply.
Many thanks for the kind info Holliday - sad about the earlier sicko.