I believe Siri will be on the iPad 3. For those who say she won't be because it requires a Internet connection: most of the things you do with your iPad require an Internet connection. People understand you need a data connection to use certain features. With no Internet you can't send/receive email, you can't browse the web, you can't use google maps, you can't stream movies or tv shows, you can't stream music, etc.
It's not that big of a deal if Siri is included in the iPad 3 if there are wifi only models. People will understand that that feature requires Internet. And if they try it without Internet it will simply reply with "sorry, I cannot connect to the server" or something.
But it doesn't go along with Apple's consistent approach to things across their hardware. Having one iPad capable of Siri at any time whilst the other can't do anything if you're not near WiFi isn't ideal.
The whole point of Siri is that it is always on and available at any time, it's a personal assistant, it completely goes against the reason for its existence if it breaks when you leave the house.