@chpwn has confirmed that Aspire which is a legal port to Siri will not work on iOS 5.1.1.However, if you want to get Siri on your iPhone 4 and iPhone 3GS devices using – Spite , a cydia tweak.Spite 3.0 has support for iOS 5.1.1 and is compatible with all devices which also includes New iPad(iPad 3).
iJailbreak has come with a tutorial on how to install Siri on iOS 5.1.1 using Spite Cydia tweak.
Spite is priced at $5, hence you will have to buy VIP Repo Access. Only one time payment.
Install Siri On iOS 5.1.1 With Spite 3.0 (Guide)
1. Open Cydia on your device and add then add the following source – http://repo.technetec.com/vip.To add a source: Click on Manage >> Sources >> Edit >> Add source
2. Search for Spite 3.0 on Cydia and install the app and then reboot your device.You will find Siri on your iOS 5.1.1 Untethered jailbroken device.You will be required to add Siri proxy server to Spite 3.0 on Cydia to work.