This paper presents MobiSpray, a novel interactive art tool for creating ubiquitous ephemeral digital art. The mobile phone is employed as a virtual spray can to spray dabs of digital paint onto the physical environment via large-scale projections. The gesture-based control of the mobile phone provides a natural pointing mechanism for the virtual spray can. Experiences from extensive field use around the world testify in favor of a successful design. Most importantly, MobiSpray liberates and empowers the artist to change the environment via large-scale artistic expressions.