There's a new Siri port for your jailbroken iPhone 4, iPhone 3Gs, and iPod Touch called "Spire" from the trio of iPhone hackers Grant Paul, Ryan Petrich, and Steve Troughton-Smith. Following the news that the latest iPhone 4S ROM is no longer encrypted, concerns about distributing copyrighted code were ameloriated and so the team released the app into Cydia (concerns about whether or not using Siri on a jailbroken iPhone technically breaks an Apple EULA are between you and your conscience).
That's the good news. The bad news is that Apple servers still authenticate Siri devices based on unique iPhone 4S identifiers, so you'll need one of those before you get started. You'll also need to set up your own proxy server in order to work with Apple's servers, so this solution is definitely not for the faint of heart. That said, there may be "other solutions" being developed for that little niggle that will hopefully be more secure than previous solutions, and a full iPhone 4S jailbreak should eliminate the need for a proxy server altogether. In the meanwhile, this 100MB Cydia download seems to be the best solution out there for previous-generation devices.