I used the following dates at
http://www.kayak.com
BWI-LEB (change in LGA) 6/16
LEB-BWI (change in LGA) 6/21
US Airways Express flights. One day mileage run on 6/21 works; haven't tried any other dates yet.
Also works at
http://www.orbitz.com and prices out at $40 RT.
UPDATE 11:15am ET - Still active. Many FTers booking multiple mileage run tickets!
UPDATE 12:10pm ET - Opportunity seems to be drying up, low fares to/from Watertown, NY (ART) seems to be the only location still available.
UPDATE 12:20pm ET - Limited opportunity to Hilton Head, SC (HHH) still available...hurry!