Apple's new Siri functionality is based on an app that's been available for some time called Siri Assistant. If you have that app, enjoy it while it lasts, as it's shutting down on the 15th.
Yes, if you want to use Siri, you'll need to buy an iPhone 4S. Despite the old app working fine on an iPhone 4, you'll no longer be able to use it. Since Apple purchased Siri in April 2010, this is the company's way of nudging people who are already on board with the voice-activated assistant into using the supercharged new version built right into iOS 5.
Did you use Siri Assistant before? Will this push you to upgrade to a new phone?