Siri is the most compelling reason to upgrade to an iPhone
4S, but it's obvious that Apple
's locking the software to that particular phone in order to convince iPhone 4 owners that they just have to have the latest and greatest. Why, technically speaking, won't Siri work on other iOS devices? Looks like it's due to a choice of Apple's. But thanks to genius hackers roaming the planet, there's a new solution. Spire is a new solution that allows for Siri to be installed previously unsupported, but jailbroken, devices. Spire is a small download, but while installing it will download Siri itself (directly from Apple). The whole process uses around 100MB of data, so you definitely want to be on Wi-Fi before you get going.
Spire isn't an end-all solution, though. Apple still requires authorization to use Siri, so information from an iPhone 4S is still required. To insert this information, Spire allows you to enter your own proxy server address. The proxy workaround isn't spelled out in detail, but the wise won't have any trouble figuring it out. You'll need a jailbroken iOS device to get it going, and probably a little luck. So with that, good luck!