An interesting challenge, especially if you think a bit about what this will prove to DARPA. What we have here is an experiment to see if 20,000,000 geeks can find something faster than 2,000 intelligence analysts in windowless rooms with access to spy satellites. If you're interested, there's a picture of a sample balloon at the link above. I imagine that one found floating over a car dealership, apartment complex, or furniture store will probably not be the balloon you are looking for, although this suggests a great exercise in deception.
To mark the 40th anniversary of the Internet, DARPA has announced the DARPA Network Challenge, a competition that will explore the role the Internet and social networking plays in the timely communication, wide area team-building and urgent mobilization required to solve broad scope, time-critical problems.
The challenge is to be the first to submit the locations of ten moored, 8 foot, red weather balloons located at ten fixed locations in the continental United States. Balloons will be in readily accessible locations and visible from nearby roadways.
What makes this interesting to me is an effort I made a couple weeks ago to find an Iranian Nuke site located "SE of Qom, Iran". Using Google Earth, I found a suspicious looking site rather quickly, and when someone published a released picture shortly thereafter, sure enough, that was it. New construction shows up on satellite photos like a fly on a plate, and "new" means anything less than 1 or 2 years old, especially if no landscaping effort has gone along with it.