New billboards are going to begin popping up in France that send messages to your phone when you are in the vicinity. See this article from the International Herald Tribune.
In theorgy, the mobile subscriber opts in initially. The idea of being permanently opted in with just one SMS until I opt out scares me. Sounds eerily like the Starbuck's scenario that people have been talking about since the late 90's. Our research shows that mobile subscribers want to opt in to not only relevant products, but specific companies in many cases. See research.
Delivering value and exclusive content to the consumer is the right idea, but frequency, among other things needs to be managed. Also, interest in free digital content – especially that tied to branding – is not that high even among the younger adults.
That said, the idea of symbols, shortcodes, etc. on posters and billboards is an interesting one. I envision them more as one time opt in's for information, content (e.g., a game) or service (e.g., movie times for one movie in my zip code).