I have to agree. After a bit of patience and experimentation, I've come to the same conclusion. I bought new snow tires (Nokian Hakka RSi's) in November, and a few days later I had my 30,000 mile maintenance done. Then I noticed my mpgs dropped considerably. There were so many different factors that could have been responsible, I couldn't decide which one to "blame" it on: the tires, the cold weather which resulted in fewer autostops, my short commutes, the dealership's maintenance...
Over the past weekend, I did mostly highway driving, and decided to check the mpg, and it was back to the range I was accustomed to seeing before the onset of winter:

http://www.greenhybrid.com/compare/m...r.php?cid=5679

My conclusion is that the tires are fine, and probably will fare even better regarding mpg as they break in. The maintenance was probably fine. I attribute the drop in mpg's to the cold weather & short commutes.
I use pipe insulation now in the grille when it's really cold out, and remove it on warm days, or when I have a long trip to make (I'm afraid of the engine overheating on a long drive; not sure if I should be concerned about that or not). I check the tire pressures, and am currently setting them at 36-38 psi (max pressure listed on sidewall of tires is 47 psi).
There isn't anything I can do about the short commute. My job is close to my house, but I work at night in a dangerous area, and walking or riding a bike isn't safe, so I drive.
Overall, the car is performing well so far, and I'm glad I bought it. Even on the worst days, 34 mpg are far higher than what my old car achieved (18 mpg city in winter w/ same short commute).

If you're not part of the solution, you're part of the precipitate.