I could almost be certain that, like everyone has been saying, a jailbreak will likely be the only way Siri will run on your iPhone 4 - and it will likely run poorly compared to the 4S. This isn't bashing, it's just likely the truth.
Yes, Apple is marketing Siri on the 4S to sell it, but you have to understand they were built for one another. You can get Siri running on the iPhone 4 eventually, but it just won't run as smoothly or as Apple intended on older devices. There's no reason to get bitter - any of you folks with an iPhone 4 who are bitter about not ordering a 4S, get on Craigslist and sell your 4 for about $300 - $400, then go buy yourself a 4S. There's no reason to badmouth something that's not even released yet like anyone really knows. More light will be shed on the subject after the device release, and everyone will sound more credible.
So be happy, iPhone 4 or iPhone 4S - you still have a killer device.