There have been a lot of great tweaks that let you customize Siri since the iPhone 4S jailbreak. The best example is currently AssistanExtensions. An upcoming tweak called MySiri also looks very promising.
MySiri lets you do even more than all the other tweaks out there. You can toggle system preferences, create your own conversations and even change Siri’s background. Perhaps the most interesting thing is Activator integration: MySiri includes the ability to execute Activator actions using your own voice commands.
Today, @rpetrich, who released several tweaks like Activator or RetinaPad, got @chpwn’s and @stroughtonsmith’s port running on his iPhone 3GS. He is now working on Activator integration.
With Activator integration, Siri could do a lot of cool things it can’t do now, like opening Apps or changing Settings (ex. turn WiFi on/off). Together with HubotSiri, which adds custom commands to Siri, the functionality of Siri could be greatly increased.
However, to install Activator your iPhone needs to be jailbroken. So we will have to wait until we either have an iPhone 4S jailbreak or Siri on our iPhone 4.