1. A system comprising: one or more computers; and a computer-readable medium coupled to the one or more computers having instructions stored thereon which, when executed by the one or more computers, cause the one or more computers to perform operations comprising: training a probabilistic model using data that reflects whether, given past combinations of (i) state data that references a state of a mobile device and (ii) sensor data that is sensed by one or more sensors of the mobile device, a user of the mobile device did or did not actually initiate a voice input; obtaining, as input data, particular state data that references a particular state of a mobile device, and particular sensor data that is sensed by one or more sensors of the mobile device; inferring, based on applying the probabilistic model to the input data, that the user of the mobile device is likely to initiate voice input; and invoking voice input functionality of the mobile device in response to inferring that the user is likely to initiate voice input.
5. The system of claim 1 , wherein the particular sensor data comprises data that is sensed by a keypad sensor of the mobile device, data that is sensed by a position determination sensor of the mobile device, data that is sensed by an accelerometer of the mobile device, data that is sensed by a magnetometer of the mobile device, data that is sensed by a light sensor of the mobile device, data that is sensed by a proximity sensor of the mobile device, data that is sensed by a capacitive sensor of the mobile device, or data that is sensed by a touch sensor of the mobile device.