Just came back from a trip from Chicago to East Lansing, MI to Toledo,Ohio and back to Chicago....average mpg traveling at approx 75 mph was 35.6mpg.
I was very pleased.

I'll be taking a trip to Baltimore next week to visit a relative in the Navy. I hope to get above 40 MPG.

Here in Western WA my mileage has been creeping up to around 36-37MPG - nice to see especially with the price of gas creeping up as well.

I have seen my mileage finally head the right direction as well. The current tank is running at 36.1 and I have had a LOT of stops and starts.

With the warmer weather, my tank average since last Friday has been just above 40 mpg.
(Which is good since prices are 15 cents/gal higher than last Friday.)

Has anyone done a comparison of what MPG you get using regular vs. high octane gasoline?

I personally have no experience with this but I've read the difference is negligible. I wouldn't waste my money...

I have seen a definite increase in my Western Washington driving. I drive 100 miles per day in mixed highway plus freeway congestion stop and go. In winter I was getting 37 mpg +/- and now can easily get 39.5 mpg (by dash display).
Other things that changed over time to consider: in pollution abatement areas the gas may have oxygenated fuel to reduce smog in the winter(can give you lower mpg ) and now my car has 8000 miles on it and I think engine wear reduces friction and might give you better mpg over time.
In my observations the single biggest robber of fuel is stomping on the accelerator: either aggressively getting on to the freeway or trying to accelerate quickly going up long hills. A couple of these stunts will make ruin your per tank mpg every time!

I have seen an increase as well. I was at about 34.5, and I did the tire pressure increase. It immediately went to about 37. Now that it is significantly warmer here, I've been over 40 in mixed (mostly city) driving.

I let my wife take the car over the weekend on a trip that was about 60 miles roundtrip. When I gave her the car the mileage was showing 40.8. When I got in it this afternoon to run some errands, it was at 38.1.

DOH!!!!