Not trying to jinx this, but the last time the Nats have swept the Fish in Miami was in 2005 (September 26-28)
The last overall sweep of the Marlins was in 2007 (May 11-13)
We also swept them at home on 4th/5th/6th August 2009 - scores 6-4, 5-4 and 12-8 respectively, winning pitchers Villone, Lannan and Sosa respectively:
http://espn.go.com/mlb/team/schedule/_/name/wsh/year/2009/washington-nationalsThe current series comes practically a year to the day since our last series win over the Marlins - 7th/8th/9th May 2010 at home: L 4-2 (LP Bruney), W 5-4, W 3-2 (Clippard with both wins).