iPhone (Jailbroken) Using Siri to control your phone is super handy, but having to activate the feature every time you want to issue a new command is annoying, especially when driving. There's a way (albeit battery-intensive) to have Siri respond to voice commands all the time.
First, you'll have to jailbreak your device. Once you do that, head to Cydia and purchase "Hands-Free Control" for $2.99. Once that's installed, head to the regular Settings app and customize what you want Siri to respond to, how sensitive it is, and even whether you want it on all the time.
To use it, just say the keyword—the default is just "Siri" but you can change it to "Computer", "Hal" or whatever you want—and the standard Siri interface will pop up. This is extremely useful when you're driving or somewhere where you don't want to constantly hit the Siri button. Is it worth $3? That's kind of up to how often you use it, but because Apple's unlikely to make this a feature by default because of how much more battery you churn through because your phone's listening all the time, this is the only way to get always-on voice recognition.
Hands-Free Control for Siri on the iPhone 4S | Blackpool Creative