I've long advocated that marketers view mobile as an opt-in response device and not as an advertising medium. Let's agree on a simple truth: everyone has their phone in hand or nearby all of the time. Sad perhaps, but undeniably true.
The huge potential of mobile is captured in this infographic, courtesy of Microsoft.
By including a mobile call to action we blur the tired distinction between offline and online; between traditional and nontraditional; between one-way and opt-in communications. Is a QR-enabled print ad in Fortune traditional or nontraditional? Is an outdoor poster with an SMS invitation offline or interactive?
I believe mobile response is best used for bringing sight, sound and motion (an amino acid of brand marketing) to otherwise static media. For example, for United Airlines BD'M included an SMS response on airport banners to enable smartphone wielding business travelers to watch a video of the airline's new first class sleeper suites; for Compellent we embedded a QR code in their brochure to enable IT managers to immediately see customers raving about "Fluid Data."
The jury is out on the effectiveness of QR codes vs SMS. I believe simple solutions that require less fiddling around will always win (aka, Occam's razor). Some people may want to snap a photo of a mobile tag, but most may find it more natural to send a text – something they already do dozens of times a day. (To that end, check out Zoove.com, a service that makes it even easier to create and own "vanity" numbers which are more memorable than most short codes.)