We mentioned a few months ago that Didi Senft -- the German dude who dresses up like the Devil for every major European tour -- had built a gigantic bicycle shaped like an electric guitar. Here's moving pictures of Didi rolling. (Sweet Adidas Minretts, too, Didi!)
I can't think Didi will have much trouble with his application to be recognized as the "world's largest mobile guitar." I just can't think of that many other mobile guitars, without taking the whole "phallic metaphor" approach.