[QUOTE=jib71]I recently had the experience of going through US immigration at DUB (Dublin Airport in Ireland) prior to boarding a flight to PHL. It came as a surprise to me that I should be interviewed by US immigration officials at Dublin airport.
In most respects, Dublin airport is a total mess, but the immigration-before-boarding arrangement was a pleasure. QUOTE]

Many Canadian airports have had US immigration/customs posts for years. It's very convenient, and lets the flight go into a domestic gate on arrival in the US. Very convenient.