I haven't seen anyone here suggest lowering a cars ride height to increase MPG. I would think it would reduce drag under the car. Might not be a drastic change but it could help if your trying to squeeze every gallon. I have lowered a few cars myself although not for fuel consumption.
There are ALOT of variables that go into creating an aerodynamically efficient car. I do know the new Grand Cherokee has a system that automatically lowers it at certain speeds, but I am sure that was well engineered to know what speed the coefficient of drag lowers by lowering the vehicle. But for the most part, newer vehicles are designed to get the best aerodymanics possible.
Another thing to consider is the cost in lowering a vehicle vs the actual amount of money saved. To PROPERLY lower a car so that it stays in alignment and handles properly it costs a few thousand dollars. It can be done cheaper, but the car would not handle well and you would tear through tires due to poor camber. Thus costing even more.
If you are redoing the suspension anyway, it may be something to consider, otherwise it would take years to recoupe the cost. Kind of like trading in your gas guzzling SUV to take a $5000 loss just to get better mileage. You need to calcuate the long term savings.
Underbody drag is one of the biggest components of the total aero drag. Lowering a car helps high-speed mpg the same way that lifting a pickup truck hurts it. It won't make as big of a difference as a 6" lift on a truck, but it may be noticeable if you do a lot of highway driving (especially 75+mph, but of course you could just drive slower for the same benefit). The OEMs are unwilling to lower a car below a certain point (for ride quality and ground clearance) so there is definitely room to improve, if you're willing to sacrifice those two.
Also, lowering a car is not as simple as the tuner boi crowd would suggest, but it certainly doesn't require thousands of dollars, at least on the typical double-wishbone or macpherson strut suspension. You absolutely need to have it aligned after you lower it, and you may need to address some camber and/or bump-steer issues, depending on the platform and how low you go. If you end up with camber or toe issues, you may end up throwing away any aerodynamic gains with a poor tire alignment and the tire wear and rolling resistance penalties that result. So definitely don't think about lowering for mpg unless you can make sure to get the alignment in perfect shape afterwards.
Most European cars, even small ones, have under trays on the undercarriage to improve airflow. On a Lotus, the bottom is completely sealed with metal sheets as smooth as paper!
Unfortunately, most US cars don't seem to care about aerodynamics. You only have to look at a wing mirror on an SUV to see it's "head sized" square mirror which probably cause Lot's of drag. I also understand they don't have pedestrian crash tests in the US? This would explain why large SUV have a high bonnet line, again very bad for aerodynamics too.
I recommend lowering the car all the time. Look how low Hondas are compared to their competitors. Also, look how much lower the Cruze is compared to the Cobalt/G5.
Anyway, for the most part you want to be looking at full-kits that are designed to work together like spring/strut combinations. Eibach produces a great series that are engineered well to maintain reasonable ride quality at the same time as improving handling.
It can make a pretty good difference. Our former 2004 Acura TL had the optional underbody spoiler and was lowered about 1.5" from stock, making the air under the car, quite a bit less than a stock TL. From 75-80 mph it got incredible mileage (32-34 on flat land) considering the 26 mpg highway rating.