This is driving me crazy because I see this in notification center in a lot of a screenshots for iOS reviews:
What is the app that adds weather to the notification center?
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You need to jailbreak your ipad (use evasi0n) then load the app onto your iPad.
Follow the instructions here: http://www.sinfuliphone.com/showthread.php?p=953237
On iPad mini this worked fine, but iPad 3 it works with the caveat that the images for the multi-day forecast don't show.