Apple's revised iOS 5.0.1 update seems to have come with decrypted ramdisks, something which could make the port of Siri to other iOS devices legally possible. Before today’s firmware update, important files to get Siri working on other devices were encrypted in Apple’s file system. Decrypting them would be illegal and infringe on copyright and that is the main reason for it not being made available.
A new jailbreak mod by the name of H1Siri claims to allow a fully working port of Siri on the iPhone 4, but this one comes with a number of questionable aspects.
First of all, the package is available via a custom Cydia repo from a relatively unknown group of hackers that also houses pirated materials, so there's a major warning sign right off the bat. The biggest drawback, however, is that the port uses a custom server to process requests on the back end instead of Apple's own servers. In other words, you'll be sending your personal information to who knows where without any assurance that you're information will be kept safe.