To me, the issue of Siri being ported over is like putting a turbocharger on a VW bus. Will it work? Possibly, but why would you do it other than to say you did.
The new Siri was designed (or updated, as the case may be) specifically to work on the new dual core A5 chip that's in the 4S (and iPad 2, though it's not available there either to my knowledge). Initially it was thought to be hardware-locked to that chip, but that turned out to not be the case. Every test I've seen of Siri on the iPhone 4 has shown that it won't connect to the server. IF (and that's a big if) they can get around that, there's still no guarantee of it working properly.
I'd love to have a 4S (or even a 4), but sadly my budget won't allow it at this time. Will I run Siri on my device? No. I have enough issues with the 3GS as it is to worry about running something several generations newer than the hardware I have.
I am Jailbroken, and I'll tell you why: Customization. I wanted to add new notification sounds (now possible with iOS 5), but couldn't. Now I can. I like having my son giggle every time I get a text message, and I could do that by jailbreaking. I like having more than 4 icons on my dock, or more than 12 apps in a folder, or being able to double tap my status bar and close all running apps at once. Necessary? Absolutely not, but it's a personal preference. That's what the jailbreaking community is all about: Personal preferences. It's the same with the Android rooting community.
Don't call us all pirates and thieves. We're not. I have never and will never install ANYTHING that's pirated. Not a theme, not an icon, and definitely not an app.