Myself and Mateusz Stawecki (a Senior Mobile Developer for the OTHER media) were interviewed for the OTHER media Blog on the release of the new iPad, Siri and the Apple eco-system. You can read the blog posts here:
Adam Swinden - iOS Developer
It is only available on the iPhone 4S, as it rueqires the A5 processor. However, if you jailbreak your iPhone 4, there is a chance that it will be available via Cydia. Don’t get your hopes up for it being ported to the iPhone 4, though.