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.