Portable multi-purpose audience measurement systems, apparatus and methods are disclosed. Example methods disclosed herein include executing media metering software downloaded onto a portable device. Such example methods also include halting execution of the media metering software in response to determining that the portable device is being used for a native purpose. Such example methods further include, in response to determining that the portable device is not being used for a native purpose, continuing execution of the media metering software to monitor a media signal output by a media presentation device separate from the portable device.