Interesting.
You made me go and download my bike journal into an excel spreadsheet and play around with it.
I classify road rides as "flat" (such as my commute), "rolling" (such as my weekend club rides), and "hilly" (such as no ride I've yet done this year, but I would think the Civil War Century would fall under that category).
I then looked at total elevation gain, as measured by my Garmin, divided by total miles for the ride, also measured by my Garmin - except for my commutes, which use my Cateye computer.
My "flat" rides have an average of 46.85 feet of gain per mile, with a span of 5.5 ft/mi (Eastern shore of Maryland, anyone? ) to 56.8 ft/mi for an extended commute of mine.
My "rolling" rides have an average of 48.86 ft/mi, with a span of 41.7 to 57.0 ft/mi.
I'm no statistician.....but I'd be willing to bet there's no significant difference b/t what I call "flat" and what I call "rolling".
Which leads me to believe that - at least for me - "flat" vs. "rolling" is all in the eye of the beholder!