Netflix is perhaps my favorite use of clever signaling, as well as a nicely understandable analogy. The DVDs are the data. They are sent via mail – the USPS is the traffic channel. The website (and to a lesser degree, email and RSS) is nothing but a signaling channel: a way to indicate what data is needed, queue future data, and confirm sending or receiving of data packets.
A similar method is used when the credit card company calls you to confirm a transaction you are performing in person, or on the computer. The phone call is an out-of-channel data confirmation signal, different from the ordering and payment process you are trying to perform elsewhere. A key use of out-of-channel confirmations is for security purposes – spoofing or intercepting one channel likely does not mean other channels have become vulnerable.
One of the things that has always made mobile telephony neat to me is that it’s inherently multi-channel. The IS-95 standard – the one with which I am most familiar – uses the paging channel to send information about the network, separately and at much lower bandwidth consumption than the traffic channels. (This is why add-on VoIP systems blow your battery so fast; signaling occurs in the traffic channel, so it has to stay up all the time).
One of the more beneficial ones is SMS. We do a lot of projects that use SMS as a signaling method. Much like the Netflix example above, users can receive an SMS with a link. This can then fires off a download, a web address or an already-installed application (usually. A few carriers like Verizon don’t let most messages carry active links).
That last – coupled with Java push messaging, not requiring the user to explicitly accept the SMS – is by far the snazziest. Pretty much every application on my phone, even the “widget engines” sits there dumbly until I fire it up, and often continues to sit there until I explicitly request data. Instead, you can build a push system, and whenever a weather alert is sounded for your area, or the flight is delayed, or a co-worker loads changes to the project plan, a specially-formatted SMS is sent to your phone and the appropriate application opens right to the most relevant screen.
(As an aside, credit card companies are starting to perform a variant of the out-of-channel confirmation signaling over SMS)
Other such practices, extending these principles, will presumably emerge over time. Telemetry data (your position, and other such information) is already being sent to the operators routinely. If this back-channel signal can get used by 3rd party services, they can analyze it and send out messaging based on your current state. Most talk along these lines is for push advertising (visit the Starbucks 1 block over and get $2 off!) but most any mobile service could be enhanced by it. This is the contextually sensitive world I’m waiting for.