In a similar style to British artist, Banksy (here is a link if you've been living under a rock for the past five years), a young hacker named Skullphone has been attacking electronic billboards in Southern California. His basic concept is to hack into the software used to display advertisements on electronic displays by showing just his logo;
Although initially impressed by this feat, it turns out he simply paid Clear Channel, an American advertising company, to show his logo on ten of their billboards. However, you have to admire the concept...hopefully someone will be able to pull it off in London. If you wish to do your own advertisment hijacking, consider one of these.