US Customs and Border protection has released guidelines that beginning March 1st, 2010 will permit general aviation aircraft operated under Part 91 to preclear their US bound flights at a CBP facility at Shannon EINN. There are numerous restrictions including the requirement for additional clearance on arrival to the mainland by a US Dept of Agriculture and the potential to be recleared by CBP on arrival to the US. Here are some of the details from CBP:

Private Aircraft Preclearance Procedures - Public Private Aircraft Preclearance Procedures
Pilots of private aircraft may request preclearance of private aircraft flights from certain, specific CBP preclearance locations to a specific, designated airport in the United States.
The preclearance processing of private aircraft is accomplished through 7 steps:
1.Request for Services.
2.APIS Submission.
3.Appointment Confirmation/Notifications.
4.Preclearance Services Performed.
5.Flight Closeout and Departure.
6.Departure Notifications.
7.Flight Arrival in the United States.
It should be noted that CBP performs Point-to-Point preclearance. Unauthorized diversions are not allowed; private aircraft are precleared from the CBP preclearance port to a specific, designated airport or airport facility that is approved and capable of handling and processing international garbage as required by the U.S. Department of Agriculture.
As indicated in Title 19 of the Code of Federal Regulations (CFR), section 122.32, CBP has the authority to limit the locations where aircraft entering the U.S. from a foreign area may land. As such, aircraft must land at the airport designated in their APIS transmission unless instructed otherwise by CBP or changes to the airport designation are required for aircraft and/or airspace safety as directed by the FAA flight services.
Through the preclearance process, effective communication is critical. The preclearance port initiates and controls communication through updates with the domestic CBP offices and the pilot (or agent) requesting preclearance; ensuring that all affected parties are kept informed and appraised of the aircraft’s status.