At first I thought it was an error until I saw the fare rules-MAXjet filed a special business class fare for their Washington Dulles (IAD) to London Stansted (STN) route. Leave May 6 and return May 7, 8 or 10 and the total comes to $498.19 after all taxes and fees. Amazing! Roundtrip business class to London for under $500!!!

The airline is all business class so I wouldn't expect inventory to run out fast.

These are the terms and conditions I found:
Promotion fare is only valid for travel from Washington-Dulles departing on May 6, 2006 and returning on either May 7, 8 or 10, 2006 from London Stansted. Travel must be booked by May 5, 2006 by visiting www.maxjet.com and using Promo Code: madmax. Promotional tickets are nonrefundable, nontransferable and may not be reissued.