Here’s how to download Siri right now on your phone.
STEP 1: Download the siri.ipa file provided at the end of this post.
STEP 2: Double tap on the .IPA file and it should open in iTunes. Now just sync your device with iTunes and Voila! You just downloaded Siri..
Download Siri from here.
Now Siri’s GUI has been successfully been ported to devices running iOS 5! Get it now on your device from here.
If you cannot get Siri working for you, then you can try this download Siri alternative from here.