Siri is an iPhone 4S only feature at this time. I believe that some of the Dev teams have sucessfully ported it to other devices... but not w/o pirating issues. Those of which cannot be discussed here. Hopefully we'll see a ligit port for pervious devices in the near future!!
Maybe not exactly in a "firmware update." From reading the other threads about Siri, it seems that it's still in the Beta phase. For right now, it's only available on the 4S.