iPhones aren't the cheapest phones in the market. iPhone 4S prices start at $199 if you buy them with a two-year contract, and $649 if you'll be getting an unlocked unit without the contract. That said, it wouldn't make sense to purchase the newest iPhone if you've already got an older iPhone that still works perfectly.
One of the main selling points of the iPhone 4S is Siri. In case you haven't heard, Siri is a voice-activated personal assistant application that only comes with the 4S. You can instruct Siri to perform tasks, schedule events in your calendar, or look up information for you, all with the use of simple voice commands. Siri even has her own following, some of which came up with an iPhone case to "honor" her (see related article here.)
Many users of previous iPhone versions have made known their desires to have Siri on their devices. Unfortunately, none of the hacks, ports, and workarounds that Apple fanboys have come up with made Siri work on non-4s devices--until now, that is. The main difficulty that these developers encountered isn't with the actual application, but with the fact that Siri needs to have access to Apple's servers in order to function.
So what a trio of developers did was launch i4Siri, a free proxy server that allows Siri to function on older versions of Apple gadgets, including the iPad 2, iPad, iPhone 4, iPhone 3G, and iPod 4G.
Note that in order to install i4Siri, your device must be jailbroken and running on iOS 5.0 or 5.0.1, 5.1. There are risks involved with jailbreaking your device, so proceed at your own risk. You can access the instructions to install i4Siri on your device here.