Article here:
On the bright side... it sounds like they think they will eventually be able to decrypt the firmware files so everyone has access to the necessary Siri resources.
These frameworks and plugins that work together to build Siri are not included on other iOS builds besides the ones running on the iPhone 4S. Therefore, these files must be copied from a running iPhone 4S, or from the iPhone 4Ss firmware (.ipsw) file. The first method requires you to own an iPhone 4S to copy the files from, so it is not useful for most people: if you already own an iPhone 4S, you already have Siri. The issue with the second method is more technical: the firmware files are distributed encrypted, and we do not yet have the decryption key to access the Siri files inside of the iPhone 4S firmware file.
On the bright side... it sounds like they think they will eventually be able to decrypt the firmware files so everyone has access to the necessary Siri resources.