Thanks to Arthur Frommer for this tip: U.S. Airways is offering round trips to 17 cities in western Europe, including Paris, Rome and London, from New York or Philadelphia for $367. That includes taxes and fees.
These fares are going fast. There are lots of restrictions and you must buy by Thursday for travel ending by May 20.
I’d recommend that you read the terms closely and then use the “low fare finder” search option on the U.S. Airways reservation page to see if you can find these fares. Use LaGuardia (LGA) as your departure point from New York and page through the dates using the “3 days earlier” and “3 days later” buttons for departures and arrivals.
If you see fares of $149 each way, the total will come out around $367, with taxes and fees included, for the round trip.
Using this method, I seem to to be finding these fares for most routings, though on just a few dates in some cases. It was harder to find between LaGuardia and Barcelona, for example, than between Philadelphia and Madrid.
UPDATE: Arthur Frommer reports that Continental is matching this fare.