Jarrod Washburn is experienceing a rather out of ordinary year to date. His 3.07 ERA screams early season fluke, but his FIP has improved from the high 4s to 3.41. Sure, some of that is improved prevention of home runs, but not nearly all of it. Jarrod Washburn has certainly benefited from his improved outfield defense. Just look at his batting average on balls in play amongst fly balls and line drives.
2009: .356
2007-8: .395

and on fly balls only
2009: .131
2007-8: .179

That is a significant boon to any pitcher. But Washburn has not just been coasting on the laurels of his defense either. Remarkably, at the age of 34, Washburn has improved himself as a pitcher as well, notably upping his swinging strike rate by over a point compared to his previous years as a Mariner. Where are those extra missed bats coming from? Lefties.

SwStr% vs LHB
2009: 14.9
2007-8: 8.5

SwStr% vs RHB
2009: 4.4
2007-8: 4.9

The RHB is so small a difference as to be within the same mean based on our sample sizes, but look at the gap against left-handed batters! Further drilling down into the swinging strikes to lefties, I present this:

In 2008, Jarrod Washburn lived off his breaking ball to miss bats from left-handers. In 2009, it is a pretty even split between his cutter/fastball (19) and his offspeed stuff (22). Of the offspeed pitches, based on spin rates, it looks like seven changeups and 15 slurvey things. It certainly appears the Washburn’s slower fastball (whatever you wish to call it) and his 4-seamer are likely candidates for his increased success against left-handers.