I wouldn't count on this happening without a jailbreak..
The iPhone uses the default Weather app to grab weather and display in the notification center. Since the iPad doesn't have Weather, it wasn't included. As far as a third party app doing it.. There may eventually be an app that can give you a text version of the forecast and such, but nothing like what you see from the iPhone. When a jailbreak for the iPad for iOS 5 comes out, then, you may be able to jailbreak and eventually find a widget for the notification center that will give you the weather.