United must have a lot of empty seats on its transatlantic flights this spring. Via an e-mail to Mileage Plus members, the airline is announcing that frequent flyer tickets between the US and Europe are being discounted by 15,000 miles for travel through May 14, 2009.

Blackout dates: April 6, 2009 – April 15, 2009, both directions of travel. Tickets must be booked online at united.com (so no Star Alliance partner routings, or connections to smaller cities served by United’s partners.)

This is a great deal. The full list of European cities served: Amsterdam (AMS), Brussels (BRU), Frankfurt (FRA), Geneva (GVA), London (LHR), Moscow (DME), Munich (MUC), Paris (CDG), Rome (FCO), and Zurich (ZRH). Full terms and conditions are here.