According to iDownload Blog's Sebastian Page along with MuscleNerd, the porting of Siri to a jailbroken iPad would be unethical and certainly illegal. Since the Siri software has been left out of iOS 5, this creates ani issue with Cydia authorizing an iPad version of Siri.
Anyone hoping for a “port” of Siri from iPhone4S: pending a very low-level A5 exploit, it likely can’t be done without piracy.
According to MuscleNerd, the only way to legally port Siri to other iDevices would be to get access to the main FileSystem keys, which requires a low level exploit. There hasn’t been such an exploit for the A5 processor yet. Without access to the filesystem key, piracy is the only option.