Dan also said that when he raised it back up he didn't notice a dramatic difference. The theory is there, but when I lowered (and will lower more) I'm more worried about handling and looks and saying "oh yeah, my frontal area is less" on a somewhat incidental note.
Darin (metrompg) has done some serious testing on the rear skirts, check out his thread/website for his results (cuz I forget where to find them).
At 47ish MPH, drag starts to show up...slowing you down(more gas!).
Just to clarify,
At approximately that speed, the contribution of aerodynamic drag is roughly half the sum of forces resisting a vehicle's motion (aero and rolling resistance).
Aero drag still exacts a fuel efficiency penalty at slower speeds, although proportionately less (in other words, it doesn't "suddenly" become a factor at 47 mph). Even at only 20 mph, it represents roughly 1/4 of the sum of aero+rolling resistance opposing vehicle motion.
Also, 47 mph is quite a high threshold (where aero forces start to exceed rolling resistance); That would represent a very aerodynamic vehicle - or a vehicle with very high rolling resistance. For most, I would say the 50/50 point is actually closer to 35-40 mph.