I have seen very similar results since getting new tires. On my 07 Touring, I used to average about 48mpg highway/city combined. After about 28k, my stock Turanza's wore out, and I replaced them with the same size Potenza's. I'm now getting an average of 42mpg. It really kinda sucks, and now I wish I had gotten some Michelin Energy efficient tires. =/ I've got about 5k on the tires at this point, and they don't seem to be any better. I've read that tires tend to get better as they are worn in... hopefully they do get better, since they're 60k tires, they might take a bit to break in.

The problem, from what I've heard, is that tire companies don't rate "rolling resistance". So, while Toyota probably figured out that the OEM Turanza's were really good as far as rolling resistance, they suck for longevity. The Potenza's are much better tires as far as handling, and ride, but they "stick" to the road more, and therefore make the car work harder to roll them on the road.

Biggest problem here in Oregon is it's nearly impossible to get Ethanol free gas. Ethanol really kills mileage, and it makes it really hard to figure out whether my mileage should be higher since people in other states aren't necessarily forced to use Ethanol'd gas.

As far as the Shell v. other gas, I haven't seen any difference myself. I might try switching back to Shell, and see what that does. Right now, I'm on Chevron. I've also seen people that suggest putting a tank of Premium gas through the prius every so often to clear out the injectors. Doing so doesn't seem to have made a lot of difference for me. =/ I've always used either Chevron or Shell gas (I never buy cheap gas), so I don't think this idea does much in this case.

Good luck everyone! Winter sucks for the Prius. Check out Priuschat.com, there are all kinds of options to help. From a block heater, to blocking part of the radiator grill with some pipe insulation to keep the car "warm".