Grant Paul (chpwn) has posted a new tool for Jailbroken iPhones that allows Siri to run on pre-iPhone 4S devices. The tool (called Spire) gets around any copyright issues by downloading Siri from device images housed directly on Apple's servers and installs them on your device. The download is about 100MB, so a Wi-Fi connection is recommended.
It's not quite as simple as that, however. Siri still requires special authentication when connecting to Apple's servers. For early testers, this has meant pulling the authentication key from a valid iPhone 4S. Not only that, but the key must be regenerated every 24 hours.
For Spire users, that means setting up a proxy server, and connecting an iPhone 4S to it regularly.
However, Spire is not a complete solution. 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. By using this (ancient) SiriProxy fork, you can setup a proxy using your own iPhone 4S to insert the needed information reasonably easily.
So, it's not quite as plug-and-go, but it's a step closer to that reality, if you are desperate for Siri -- and have an iPhone 4S at your beck and call. The proxy workaround is required while we wait for the iPhone 4S jailbreak that should simplify the matter somewhat, but even then you'll still need regular access to an iPhone 4S-generated authentication key.