One important role that I see for the future of the cell phone is as a bridge between the physical world and the digital one. An aspect of this will be the ability to leave a virtual message or tag attached to a place or object, for others to find and read/see.
For instance, London has its famous Blue Plaques, which are plaques (blue ones, funnily enough). These affixed to houses that were the residencies of famous people, like the plaque at 221b Baker Street celebrating Sherlock Holmes(actually, it's on number 221, as 221b is as fictional as the character). If you visited it, your phone could give you access to a wealth of information "hidden" virtually at the site. This could range from more biographical information on Sherlock and his sidekick, Dr Watson, to the complete works of Sir Arthur Conan Doyle.
This potentially brings the Wikipedia into the physical world, enriching it beyond measure.
Obviously, you wouldn't need a visual plaque to tell you that there was information available, providing that your phone could alert you to it. And that the phone was instructed as to the type of information you would find interesting, and more importantly, messages you didn't want to get. One man's valuable information is another woman's spam.
Such tags wouldn't have to be purely informational. They could be commercial too - maybe offering you a discount off a Sherlock Guided Tour or a coupon for a local bookshop. People have been talking about Location Based Marketing for years, but this could make it reality. Clearly, in order to work, this would have to be strictly opt in and policed mercilessly.
Messages could also be personal, although there would need to be a clear advantage over direct contact by sms or voice. But an example might be a scavenger hunt, where people followed clues from location to location - like the exploding sport of geocashing works today.
Or maybe I want to set a location based reminder to myself that the next time I walk down this street, I need to visit a shop which is currently closed. Yes, there are other ways of doing this, but location-based alerts might offer a better solution in specific instances.
This type of system doesn't need to be restricted to a place. In a book shop, you could scan the barcode with your phone or input the ISBN code into (say) a Java application on your phone and see the Amazon or epionions peer reviews on the book.
Or in library (remember those?) or art gallery, you could see what previous readers/visitors thought or get together online or physically to discuss it, if you wanted.
Or scan a product in the supermarket to find out more about the product ingredients, including what that innocuous looking E number actually means.
As you'll appreciate, the list is endless (including many applications for sex and porn, as many reviewers of this article noted), as well as being transformational.
This kind of technology is now in the process of being deployed, whether we're talking Siemens' Digital Graffito or scanning barcodes.
There are clearly many issues to overcome to make this a reality, not the least of which is who polices and controls this virtual content. For instance, what's to stop an unscrupulous restaurateur from posting a fictitious and favourable review of his establishment? But Wikipedia has demonstrated that there are ways of managing this kind of issue most of the time.
However, if this system is introduced, I believe that it will enrich the world around us in a non-polluting way and make the world an even more fascinating place.