I meant to post this a while ago and forgot about it. A huge digital arrow sign was installed over London to promote Nokia's free Ovi map navigation system. It was up and active for two weeks. People could interact with the sign by sending it a message from their phones. See the videos below for more details!
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-My Nokia Blog
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The World's Biggest Signpost from adghost on Vimeo.

Notice how the people who see the signpost use their mobile phones to take pictures and share with their friends.

I found a similar video on the Infosthetics blog:


"This gigantic, interactive signpost sponsored by Nokia Ovi Maps in the form of a dynamically rotating electronic LED screen allows passers-by to send in their favorite location and coordinates via text or email. The giant pointer, hung on a 60 ton construction on height of 50m, then automatically rotates to the given direction and displays the submitted description to the world."