The iPhone 3gs for me still has a lot of shelf life left to offer. Whilst the PC and consumer world focuses on cores and ghz, the 3gs still does a great job at emailing, texting, phoning and web browsing and now it appears, running Siri. People can blather on about having more cores and more speed but what about the software and that's the line that Apple marketed on the iPhone 4s and most would argue it's USP (unique selling point). People can predict, or want, that Siri will come down the line to the 4 and iPad 2 but till then it's being hacked to within an inch of it's life.
Like most new hack the bugs are still being worked out from the system but WOW, come on this could be something major if it gets released... but that's probably not going to happen.
As @stroughtonsmith and I demonstrated a few days ago, it is possible to run Siri on iPhone 4 and iPod touch. However, as we are currently unable to distribute the port or the procedure we used, I think I should at least explain tthe reasons why that isn’t happening