Mine has definitely gone up as well. I purchased the car with 7500 miles and I was getting 15 - 16 mpg during my rush hour commuting. Now at 15k miles, I'm getting 17 - 18 for the same commute. I managed to squeeze out 24 mpg on a highway trip, although I had to keep the max speed to 75mph for the most part to get that.
I have attributed some of this gain to the fact that I bought the car in July, so I was running the A/C all the time when I first got it. I guess I'll know for sure next summer.
Keep in mind that the new gas with 10% ethanol will yield a little less mpg than the old stuff. I believe it'* something like a 5% drop.