Along with the news of an untethered jailbreak for older iDevices came the ability to install Siri on any jailbroken iDevice running iOS 5.0.1 thanks to a handy installer called Spire. Of course, installing Siri is the easy part. Getting it to work is another story entirely.
Actually, installing it may not be the easy part at the moment. We've been trying for most of the day but can't get the BigBoss repository to load in Cydia or even manually install using Spire .deb package file (which you can download here). As a result, we haven't been able to test Siri on our older iDevices but can tell you a little bit about what you can expect if you're able to load the repository on your jailbroken iPhone, iPad, or iPod touch.
Basically, you're going to use Spire to download Siri onto your device. The installer app is small, but what it will download is not. It'll take up about 100MB of space so you might want to have a Wi-Fi connection when you're initiating the process. Once the install has finished, you'll need to reboot. (In some cases, you may need to perform a tethered boot so be prepared for this.) All of that is the easy part.
It gets more complicated when you have to help Siri connect to Apple's servers. If Siri is not on an iPhone 4S, it knows and won't succeed in getting any response from Apple. As a result, you need to use a proxy and have access to an iPhone 4S. This means grabbing SiriProxy and setting it up to use an iPhone 4S' authentication tokens. (More details here.)
At the moment this process is kind of a pain in the butt. As chpwn points out (he's one of the developers), we'll likely see some pay services and alternative servers to use with Siri in the near future. For now we suggest waiting until using Siri isn't such a hassle and you don't have to rely on someone else's iPhone to make it work.