Do you mean continuous animation in the sense that you can slide the weather widget to the side to reveal the week forecast? If so, that's been there since iOS 5.0
I don't see this feature you speak of.
Today when I look it shows rain, but the same as before, static image of rain.
Last night it showed fog or clouds, and the graphic itself was continuously scrolling horizontally. Really nice looking effect. Honestly maybe that particular one was already like that and I just never noticed.