Noted Jailbreak developer chpwn has posted a statement on the state of the iPhone 4 and iPod touch Siri port and further explained why it's not currently being distributed.
Many people have managed to display the Siri UI on the iPhone 4; it is, in fact, reasonably trivial with access to the files copied off an iPhone 4S (as explained above). But only Steven and myself — yes, I know there are others that claim to have: I’ll tell you this, they haven’t ;) — have managed to make Siri successfully contact the Apple servers and receive responses. Why? Here, the answers become slightly more murky. Partially this is because I don’t want to reveal too much about the procedure to try and ensure that you all will be able to use it in the future, and partially because it requires a jailbroken iPhone 4S, something which is currently not publicly available. Anyway, the general gist of it is that you almost certainly need the access provided by the a jailbreak to extract all of the information necessary to get Siri working on another device, and that’s not yet availble. (And, no, I don’t know when it will be. You can follow along with me while we wait, though!)
Hit the link below for all the details.