Looks like the guys @i4Siri found a way to port Siri to iPod Touch 4G devices that processes queries through their own server with a little help from the Google API. And unlike other Siri port servers out there, this one is going to be free but a little donation will definitely not hurt, plus donors will get first dibs on the server once it goes into beta testing.
You can check out this twitter list for updates on their progress.