Spooks
This would be a mash up, and would produce an online/real life live action role-play game platform.
What you do with it from there is up to you - it could also be used as a platform for, a local business directory, geo-directory enquiries, tour-guide service, etc.
This idea was inspired by the BBC series 'Spooks', which shows a 'Controller' online, directing people in an actual city. The Controller sees dots on a wireframe model of a city, and talks through their mobile phones.
This would require a VRML model of the Central Business District of a City or wherever else you wanted to play.
Activate the players 3G phones to become 'transponders'. Overlay the GPRS-determines location of phone in the real world, onto the model world.
The Controller could fly up to the avatar/ dot/ representation of a person in the real world. Rather than dialing their number, click on them to place a mobile phone call.
The game would comprise coordinated action between real world players and online players. Use your imagination.
Example 1: Team A (10 people) have to covertly follow Team B (3 people) who are trying to lose them.
Example 2: Treasure Hunt.
Example 3: Play Monopoly with the real City.
Example 4: A more intricately and creatively directed flashmob - go for something more akin to a Mexican Wave than mugging a merchant.
Once this platform was set up with, for instance, a Virtual "London", you could enrich it with tagging the GPS location of local merchants (monetised), and use it to provide Tourist Information, Geo-Directory Enquiries, etc. This creates a whole business model.
Is that creative enough? I have other ideas, but lets see how this one flies first, shall we?