Mobile human computing

I am a big fan of Luis von Ahn's work on human computing. The idea of human computing is to harness the idle human brain power to perform computational tasks that so far have eluded the computers, such as image labeling (see google's image labeler) and recognition of old text (check out reCaptcha). The clever part of von Ahn's approach is to engage the human brain through an entertaining game or as a by-product of other necessary tasks.

I believe that this approach can be extended to the mobile phone platform. With smart phones like the iPhone, the location and time of the carrier are known, and these information should be valuable for designing location- and time-specific computational tasks. Another advantage of using smart phones as a platform for human computing is that these phones are almost always accessible at idle times - my colleagues invariably play on their iPhones when the meetings get boring. 

What can be accomplished using smart phones and idle human time? I have not come up with a specific application. But here are some general, and obvious, thoughts. Each human should be presented a simple task, which should not require expert knowledge or special skill, and should be fun to perform. The task should be easy for normal humans but difficult for even the most advanced computers, such as to observe and describe one's surroundings, or to identify a person. The outcome of all these tasks should then be combined in a way to solve a complicated problem. In order to take full advantage of the mobility of smart phones, the problem that we try to solve should ideally involve the locations and times at which the carriers perform their tasks.

Ideas?