Apple will find a way to block unauthorized Siri installs from using their servers. All Siri requests must contact Apple for voice recognition and processing.
I would not say that Siri is the only selling feature of the iPhone 4S. If you have an iPhone 4 then you most likely would not upgrade to the 4S unless you have a money to burn or NEED a legit factory unlocked device. If you have a first gen, 3G or 3GS then the 4S is a huge upgrade.
I just upgraded from a 3GS to the 4S and am blown away by the speed, camera and convenience of Siri. Also the 3D graphics on new games like GTA3 and Infinity Blade 2 are only available on A5 processors, same with OS-wide AirPlay video mirroring. The iPhone 5 is 1 YEAR away and I wanted an upgrade now, waiting was not an option for me. Also check out the following links, side by side with the 4, the 4S is faster in every task, also the new camera takes great video, image stabilization is a great upgrade.
Speed Comparison Video of iPhone 4S and iPhone 4 - Mac Rumors
A Variety of iPhone 4S High Definition Sample Videos - Mac Rumors