Sorry if this has already been posted. For newbie like me.

How to install SIRI in iphone 4/3GS, ipod touch 4G/3G & ipad 3/2/1 running on ios 5.1.1

a) All you need to do is first jailbreak on 5.1.1
b) and then open Cydia, Hit “Manage”, “Sources”, “Edit”, “Add”
and add the following source: “repo.siriport.ru”.
c) Once that finishes, open that source and install “Siriport Original”.
There are 2 packages, 1 for your iPhone/iPod & the other for your iPad.
d) Exit cydia.
e) Now Go to Settings and open the Siriport tab down below, then Click

“Install Certificate”, this will take you to a new page where you can install
the certificate from.

Once that is finished restart your iDevice. Once back up and running hold
the home button and keep talking to siri, eventually She will start working
and you will be good to go!

Source: Everythingapplepro.net (Youtube)

Since its forbidden to post links, that's why i made this one. And additional,
as they say, sometimes it takes few minutes before it works, i was just
lucky that it work immediately after the installation. But if SIRI says
something like , "sorry i cant answer right now", just try and try, every 15
minutes intervals, say "Hi SIRI", because many people are using SIRI. It
worked for me. Just sharing guys. Good luck.

PS: It only needs internet to work.

-iPhone 4 32 GB 5.1.1 (9B206) Baseband 4.12.01 (SOFTBANK Japan locked) - JB Snowbreeze 2.9.5