Yep, I'm one of those guys that keeps track of these things. I keep a book in my car and document every maintenance done to it, the date, the mileage, and I've kept track of each fuel up for the first 100,000 miles and calculated the miles per gallon per each fill up.
I purchased my 2006 N/A AT in April 2006, and it took me 3.6-years to get to the first 100,000 miles since I put ~65 miles on my car daily going to and from work. I've changed the oil at regular intervals, replaced my engine air filter every 10,000 miles, use practically the same brand of gasoline, performed the specified 30K, 60K, and 90K maintenance.
I've been entering the fuel data (date, mileage, calculated MPG) into an Excel spreadsheet and made an MPG vs. Time chart. My overall average is 26.8 mpg (), lowest monthly average is 24.0 mpg (March 2009), and highest monthly is I used 28.9 (July 2008). You can see seasonal fluctuations in mpg. The vertical lines mark each year anniversary from which you can see an overall yearly performance, and I see an overall decreasing trend in mpg.
I can imagine it will stabilize at some time. </fingers crossed>