UPDATE: iOS 6 will add Siri to the new iPad.
The third generation iPad (or iPad 3) does not feature the personal voice assistant Siri.
Siri requires an internet connection to work, and unlike the iPhone 4S, the iPad is not always guaranteed to be connected to Wi-Fi or 3G. Even the 4G iPad models are not guaranteed to be connected since the iPad is pay as you go. Meaning it isn’t linked to a wireless carrier contract like the iPhone. Since Apple can't guarantee that Siri will be able to connect to their servers, they decided not to add it to the iPad.