Here's an interesting patent filing: This one that looks to be using a motion sensor to wake a device. In this example, we can't be sure, but it sounds as if Apple has been or is exploring the idea of having a device wake upon detection of motion.
The motion must go beyond a certain threshold, according to the patent filing, but we don't know if that threshold can be met by simply carrying the device in your pocket and having it turn on as you walk around (we are sure Apple wouldn't do that) or (more likely) when it detects a huge motion such as your arm raising the device up near your face, it will automatically wake. This sounds possible, and maybe a bit practical:
Systems and methods for processing motion sensor data using various power management modes of an electronic device are provided. Power may be provided to a motion sensor during a first power mode of the device. In response to the motion sensor detecting a motion event with a magnitude exceeding a threshold, the sensor may transmit a wake up signal to a power management unit of the device. In response to receiving the wake up signal, the power management unit may switch the device to a second power mode. The device may provide power to a processor and load the processor with a motion sensing application when switching to the second power mode. During the second power mode, motion sensor data may be processed to determine that the motion event is not associated with an intentional user input and the device may return to the first power mode.
Upon reading, you will see that it does have a mechanism that will put the device back to sleep if it figures out that the motion was unintentional.