BA 318 at Shannon
The BA 318's clear US immigration and customs at Shannon. Most Flights from Dublin and Shannon to elsewhere in the US (Delta, US Air, Continental, American and of course Aer Lingus) such as Newark, JFK, Philadelphia, Charlotte, Orlando, Chicago, Boston and Atlanta do this and I can tell you it is quick and saves a lot of time stateside. There are a couple of exceptions - afternoon flights from Dublin e.g Aer Lingus to Boston at 2pm as the clearance facility seems to be morning only.
Immigration has been this way for donkeys years but coustoms only added at Shannon in 2009 and Dublin recently with the opening of terminal 2.