We know there's a huge demand for Siri from iPhone 4 and 3GS owners, and Apple has yet to annouce any plans for allowing the beta service to run on these devices, along with iPod touches.
See also: How to control your iPhone 4S using Siri
Don't fret, though, as you can have Siri on your iPhone 4 today, and it's all legal and above board. It's also free, and it works. Sound too good to be true? Well, there is a catch: you need to jailbreak your iphone first. If that's meaningless to you, read our full guide to jailbreaking. This will take you step-by-step through the process.
Once you've jailbroken your phone, fire up Cydia (the alternative to the App Store) and search for Spire. Spire downloads the files required to install Siri on your iPhone 4, 3GS or fourth-generation iPod Touch. As with all Cydia installers, you'll see details of the files being copied along with a progress bar.
Once installed, go to the normal iOS Settings app. In General you should see a new Siri option. Tap this and slide the toggle to enable it. Return to the main Settings menu and look for Spire. It will be in a block of settings for Cydia apps. Enter https://i4sirifree.mooo.com as a Proxy Host (note the 's' at the end of http, and three 'o's in the URL).
UPDATE: 30th March 2012. The i4Siri servers have been down for over a week now, but we've found an alternative that appears to be more reliable, and has extra features to boot. So, for the Spire Proxy Host, enter the URL https://connect.grwh.net:1001. There are 10 servers, so change the last two digits to connect to a different server, e.g. 07 for server 7.
Finally, in Safari, browse to http://i4siriserver.com/installcertificate.html and click Install when prompted. UPDATE 30th March 2012: To use the Green River Siri Server, don't use the previous link for i4Siri. Instead you need to install the appropriate certificate from this link in Safari on your phone. Don't worry that it's not trusted - this is because it's not authorised by Apple.
You can now long-press the Home button to launch Siri, and ask it to do something. If you find that Siri isn't understanding anything at all, go to the Siri settings in the Settings app and change the language to English (US).
Unlike previous attempts to get Siri working on non 4S handsets, this one doesn't rely on procuring authentication keys from an iPhone 4S. Instead, Green River's servers use Google's speech recognition service instead of Nuance's as with 'real' Siri.
Bear in mind that personal information is sent to (but not stored on) the proxy server. Also, don't expect the full set of Siri functions to be available. Reminders, for example, are still being developed. Interestingly, though, location services do work: something that even 4S owners don't have in the UK yet. For more information, visit the GRWH siriServer website where you can also buy private slots on the server for approximately £3 per month or £6 per quarter. This gives you better service with more commands including the Wolfram Alpha plugin so you can ask Siri questions such as "How old is the Queen of England?".