Developers claim to have cracked Siri's code, which was created to work only on iPhone 4S, and have made it work on other iPhone devices as well as other platforms like Android.
Slashgear has the story:
Applidium developers were able to decipher Siri’s security protocol by setting up their own custom SSL certification authority and adding it to their iPhone 4S to find out the commands that Siri sends to Apple’s official servers. They have released the access tools and given a full rundown of their process.
In theory, this hack could allow Siri to be used on an iPad, a Mac, or even an Android device. But the catch is that it still requires the UDID of an iPhone 4S, which lets Apple’s servers believe that the device is authorized. But if Apple decides to block your UDID for suspicious activity, that would also kill your iPhone 4S.
Applidium developers also described learning that Siri has a lot of back-and-forth "communication" with the Apple server that could pose problems for those trying to get the Apple personal assistant on their devices.
Android already has its own version of Siri called "Jeannie", a free app, and a pay-for app called Voice Actions Plus that is similar to Siri as well. Watch how Jeannie works here: