Unless your name rhymes with "Anksy" or you've got the chops to do burners like this, you might want to consider a less...permanent means of expressing yourself in the public space. The Light Spray Can might just be the answer.
Instead of spraying paint, the Light Spray Can emits a high-intensity beam of UV light. When combined with long-exposure camera shots, you can achieve stunning light-painted results. In addition to the spray-can shaped UV light, the kit includes letter stencils and a UV-sensitive backdrop that holds the light longer, allowing for more intricate works. The Light Spray Can runs on a single AA battery and retails for the equivalent of £24 at Photojojo. With international shipping thrown in, expect to pay £34 all-up. [Photojojo]