1. A method comprising: registering, by an application software module, to receive information from a portable device operating system service, the operating system service using audio fingerprinting to identify audio captured by a microphone of the portable device, the operating system service outputting information identifying the captured audio; receiving digital watermark data steganographically encoded in the captured audio; and presenting to a user a commercial offer that is based on the identification information, and that is also based on the decoded digital watermark data.