There are actually some grey areas as to which laws apply on aircraft at specific times (or, more accurately, in specific locations). For example, the US authorities claim (with a certain amount of justification under international law) that US laws apply on all aircraft (irrespective of the airline) destined for the USA, from the moment that the doors are closed. So, as far as the US authorities are concerned, US laws apply to the passengers and crew on a BA plane, destined for New York, while taxiing prior to take-off at Heathrow. However, if an incident occurred at that time, it's likely that the plane would be delayed so that the UK police could make an arrest and bring the offender before an English court.
In many case where disorder occurs on an aircraft, it's in international air space, so the pilot will divert to the nearest available airport. Under international agreements, the laws of most countries permit prosecutions of persons who're accused of endangering an aircraft, even when the alleged offence did not occur in the air space of that country and even though the airline might not be registered in that country.
Chris
(PS: Watch out for an answer to this question from Clanad. He's an airline pilot so he ought to know!)