GraffitiWriter is a tele-operated field programable robot which employs a custom built array of spray cans to write linear text messages on the ground at a rate of 15 kilometers per hour. The printing process is similar to that of a dot matrix printer. GraffitiWriter can be deployed in any highly controlled space or public event from a remote location. - GraffitiWriter Website
I was more focussed on Graffiti Writer for my own personal use, but imagine the implications for guerilla marketers!
If your looking into buying, make sure to check out StreetWriter, because it makes writing visible from low flying aircraft:
The expanded width of StreetWriter allows for messages and simple graphics that are legible from tall buildings and low flying aircraft and is capable of rendering messages that are several hundred feet in length. - GraffitiWriter Website