Patently Apple provided some insight into an Apple patent application published today by the US Patent & Trademark Office for "audio hyperlinking." These hyperlinks can be audible or inaudible, and can be played back on podcasts, TV shows or in-store speaker systems to send messages to apps on your iPhone.
As Apple notes in the application, hyperlinking has been limited to textual documents like web pages, and this process provides a way to allow hyperlinking of non-textual (video / audio) materials as well. As an example of how this could be used, imagine watching a recording of TUAW TV Live and having inaudible cues from the recording link you to the product pages of items we're talking about on the show.
A hyperlink in one audio stream can even link to another audio stream -- for example, a hyperlink in a podcast could link to an advertisement audio stream that doesn't need to be embedded in the actual podcast, then return to the podcast once it has completed playback.
As always with patent applications, there's no guarantee that Apple will ever use this technology in any forthcoming product or service.