I'd be interested in finding out more about this. My parents (who live in NH, which gets its fair share of cold days) have a Prius and don't notice any mileage hit in the winter - in fact not running the A/C seems to help things more than cold would hurt. But there's definitely a difference between a hybrid and a full electric...
I suspect the hit is less for a hybrid, because in a regular Prius, the battery is basically used as a buffer, not a giant energy store. I have a plug-in Prius and my "pure EV" range (this is how far you can go without firing up the gas engine at all) is significantly shortened by cold weather. I haven't done any obsessive tracking of it and I've only been through one winter, but I'd say it's somewhere around 25% reduced capacity in the winter.
You're right, a plain hybrid is substantially different here.
A regular (non-plugin) Prius will go maybe a mile on the battery alone, and only in the right circumstances. Acceleration on the battery is poor, and battery-only mode is limited to 40-45MPH. It'll start the engine for you on demand, so this is no problem when driving, of course, it just illustrates that the battery performance isn't all that important.
Depending on how you drive it, cold weather can hurt gas mileage. If you're driving in a way that the engine doesn't run much, then it may have to run the engine more to generate heat for the heater. However, if you're driving in a way that runs the engine a lot anyway (highway driving, decent amounts of acceleration hard enough to require the gas engine, etc.) then you get the same kind of "free" heat as a normal car.
One interesting point: you mention acceleration. The battery-only acceleration on the plugin prius is actually pretty acceptable. I recently rented a regular prius and my habit of accelerating as fast as I can without busting out of "eco" mode meant I was just crawling embarrassingly slowly, whereas in the plugin, you tend to outrun most average drivers if you do that.
The battery makes a big difference. Since it's just a helper on the regular Prius, it doesn't have to be all that powerful, and I think it's only rated for something like 30-35hp. On the plugin, it needs to be able to drive the car on its own in a reasonable fashion, so it's much more powerful, as well as the obvious thing of having a higher capacity.
Driving my Prius v, I'm battery-only basically when cruising on surface streets, sitting idle, or maneuvering around small, slow residential streets. And maybe, maybe if I'm accelerating away from a stop on a decent downhill slope with nobody behind me.
Winter gasoline is 2-8% less efficient than the summer blend. My Prius's mpg, that I've never reset since buying the car, goes down slightly in winter, and back up when the switch the fuel back, only by a few mpg, and less now as it's averaged over many more miles. Now at 49.1 (uk gallons)
