Can SIRI be put on an iPad2 ....
Doubt it. That's Apple's baby, not an app.
It was an app until the 4S came along. Apple bought the company, repackaged it around the iphone release, and then pulled it from the app store.
There are no technical reasons it couldn't be made available to the ipad2. Apple of course does have business reasons for doing so, which probably do not change. But they did sell an awfully large number of 2s and not all of them are going to upgrade. Since search drives a lot of business, esp for google, at some point they may want to get those tens of millions using Siri.