Well, not so much an idea as improving upon somebody else's idea.
http://www.boreme.com/boreme/funny-2008/chase-balloon-berlin-p1.php
These guys floated a Handycam using helium balloons to get aerial shots of Berlin. They had to go haring after it on bikes in the pouring rain. Seems to me if you shot the video on an N95 with the GPS on and running "3DTracking" or similar, then you could just watch its progress on a laptop, and go and collect it when it came to rest. Trick would be to (a) find it before the battery ran out and (b) find it before somebody else nicked it.