Here is how to install siri on the iPhone 4, iPhone 3gs and iPod Touch 4G, iPad

To install siri right on your iPhone, iPod or iPad you need to jailbreak your iDevice.

Steps for jailbreak follow here http://eddiebedi.blogspot.in/2012/03/how-to-jailbreak-iphone-ipod-ipad.html 

Step 1: Add the Source to Cydia  -> "repo.siriport.ru" without quotes.

Step 2: After adding the source, go to SEARCH tab and install  “Siriport”

Step 3: Reboot your iPhone

Step 4: Install the Certificate from the website."http://www.siriport.ru/1.crt "

Step 5: Reboot

Step 6: Activate Siri and say “Hi Siri” Siri won’t work, wait 1-4 minutes and try again. (Say “Hi Siri” again)

Step 7: Repeat step 6 until Siri is activated