Hand-Free Control is a new tweak that will let you activate
Unfortunately, Siri activation will have to wait until the 4S is released; however, for the time being you can set the tweak to activate the normal Voice Control feature on pre- 4S devices…
Hands-Free Control provides a continuous listening service on your device to activate voice functions, even while in sleep mode. When you speak the keyword, “Siri,” the Siri service (on the , once jailbroken), or Voice Control (on , 3GS, iPod 3, and iPod 4) will be activated without pressing any buttons, and then wait for further voice input. Normally you have to press and hold the home button for several seconds to activate these voice functions, but now with this app these features are completely hands-free! Just say “Siri.”
Sensitivity adjustment to the key word, as well as other configuration options, are provided in the Settings app under “Hands-Free Control.” It also works with headphones and bluetooth devices. Because Hands-Free Control runs continuously in the background, noticeable battery usage is to be expected while enabled. To disable, turn off in the Settings.
No icons are added to the home screen. You can configure Hands-Free Control options from your Settings app.
Hands-Free Control can be purchased from the Store for $1.99.
Read More [via iDB]