Interesting concept and great fun at the same time. A Canadian artist has drawn a 55ft image of Wally (Waldo if you're North American) on a rooftop in Vancouver. It will be visible via Google Earth once Vancouver has next been passed over by Google's imaging eqpt. You need to know the location and you need to wait for anything up to 3 years to find it.
All the details are at her Waldo blog.
I'm sure a Marketing Dept could adapt this concept and use the amazing service offered by Google Earth for other fun activities.