Well, a long-standing question has finally been answered...  We always used to wonder why I got 28-30mpg out of my 2g Eclipse, while my other friends were only getting 22-25mpg.  The assumption was always just that it was because I drove slow (which is a legitimate thought, because I always drive at or below the speed limit).  Since converting the car from the ancient aftermarket HKS VPC piggyback fuel computer back to the stock MAF, my mileage has plummeted to match what they get...  Interesting that not only does the aftermarket computer unleash a good chunk more horsepower, but ALSO gives a sizeable boost in mileage per gallon of fuel!

Oh well, while it's sad to lose the horsepower and the gas mileage, overall I'm happy with the switch back to stock.  The car idles more consistently (it can now idle all by itself when the temperature is below freezing) and the cruise control actually maintains speed properly now (The VPC presented a MUCH different steady-speed vacuum level that gave the cruise control fits and made the car do weird surging things).

Case closed on the great mileage mystery!