Immediately after the release of the iPhone 4S, many users were quick to ask when they would be able to install the personal assistant on their old iDevices. Then the hackers from the Dev Team, Chronic Dev Team, etc. said that this would be possible and after a few months it came true. Users can install Siri on their own terminals using Spirals or AssistantConnect but unfortunately the assistant is semi-useless because it does not work without a server to interpret the words and commands spoken by the users.
Although there are many servers that allow users to use Siri through the ports available so far, unfortunately this method requires compromising the data from your terminals because those servers will be able to copy your contact book, emails, phone book, etc. The problem is that hackers have not been able to find a method to connect an iDevice to Apple's servers without a certificate made on an iPhone 4S terminal and as things stand this will not be possible in the near future because of the way he thought Apple system.
In short, the possibility of using Siri on an iPhone 3GS/4 or an iPad tablet seems impossible without discovering a method to falsify the certificates of the iPhone 4S terminals. Maybe the guys who are supposed to develop these solutions take advantage of the fact that many of the major repos in Cydia have Siri servers and make good money from them, but whatever the reason, I don't think you will be able to use Siri without problems on old iDevices ever.
