As robko said, there was a change in cover handling with FW 2.3.1. The change is how images are resized when used. With previous firmware, the aspect ratio was changed to 3:4 when the image was resized to display. But with 2.3.1, the aspect ratio is not changed when the image is displayed.
The difference that Geoff is seeing is how calibre copies covers over. When it does this, it resizes the covers to three defined sizes. But it doesn't respect the aspect ratio of the covers in the metadata. So, if you use the cover copying options for the driver, they become distorted if they don't happen to have an aspect ratio of 3:4. It is on my list of things to fix. I don't use the options, so I never see it to remind me. And not many people have noticed