I’m not sure how long this will last before Apple plugs this one up but according to The Future Report, developers at Applidium have cracked the protocol that Siri uses. This would allow anyone to develop an app that would communicate with Siri.
This was done by faking an SSL certificate and iPhone 4S like headers and Siri accepted the information. When you talk to Siri, the information is a compressed into an audio file sent to their servers. The server sends a iPhone 4S identifier to validate the device then sends the answer back.
They have released their documents and the source code so people can jump right onto developing for Siri. This information being out, though, I can see Apple shutting something down to prevent unwanted Siri traffic offer their servers.