Currently there's only a 3x weekly Cargolux 747-400F flight from LUX to New York JFK plus 1-3 747-200F flights of the Israeli airline CAL.
On the passenger side, Loftleidir/Icelandair operated between 1955 (I think ) and 1999 from Luxembourg via Keflavik/Iceland to New York, starting with a DC-4, then DC-6, Canadair CL-44 (or Rolls-Royce 400 as they called it), DC-8, DC-10 and in the end a Boeing 757.
Loftleidir didn't serve any other place in central/mainland Europe in the 1950's and 60's besides Scandinavia and Prestwick, so Luxembourg was their gateway to Europe. The reason why they stopped after so many years was the huge lowfare competition from other carriers like Lufthansa, Air France, BA etc, making the travel to Luxembourg plus the additional stop enroute not worthwile anymore.
As far as I know Luxair never used the 747SP to New York, it was only used on flights to South Africa, which was also more a political move due to the fact that SAA was not that much welcome in Europe during the time of apartheid.
Luxair used not a Sobelair, but a Citybird 767 for the LUX-Newark route if I remember right. The financial results were desastrous so that the route was cancelled as quick as possible.
The reason is certainly the proximity to other hubs like Frankfurt, Paris and Brussels, but also the very small market in Luxembourg : The city of Luxembourg has only about 80.000 inhabitants, nearby Trier/Germany has about 100.000, so there is really no interesting market base for such an operation.
BTW Daniel, I don't think that Luxembourg is so boring : You get there the Cargolux fleet, the small guys of Luxair, Crossair, KLM, Sabena, BA/Cityflyer, Cimber Air but also MK Airlines 747-200 and DC-8, Atlas Air 747-400 and 200, CAL (Israel) 747-200, China Airlines 747-200 and 400, MNG Airbus A300, Azerbaijan Airlines IL-76, Aerocharter Antonov-74, etc etc. plus not to forget those aircraft that undergo maintenance at cargolux : all Corsair 747's (including the SP F-GTOM!), many 747 and 767 from Air Atlanta Icelandic in the colors of their respective operator, some private DC-8's and so on... I would really not call it boring!