Basically this is just a trick that avoids fuel surcharges of about $230.
Start at Kayak.com, enter JFK as your starting city and FCO as your destination. Price will come up at about $526 for multiple airlines/1-stop in each direction.
What you’re looking for, and in most cases this will come right up, is a combination of United (JFK-IAD) and a United flight sold as an Air One (Lufthansa partner) codeshare (IAD-FCO).
When it comes up this way and you click through it takes you to Cheaptickets which gives you a message, “Great news! We found an even lower fare for this itinerary. Act fast to take advantage of this fare before it expires. (Message 102b)” The price falls $230 — down to $296.
This trick also seems to work for San Francisco-Milan (price comes up at $705 for multiple airlines one-stop, prices out on Cheaptickets for $495) and San Francisco-Rome (winds up @ $506).
Presumably will work for other cities as well, anything that lets you connect through Dulles onto a United non-stop to Europe where you can book the United transatlantic flight as an Air One codeshare.
Note that while these flights earn United miles, they are not upgradeable.
(As discussed on Flyertalk.)