Vladimir Umanets painted a message onto a Mark Rothko painting in the Tate Modern, and has just been arrested - bbc.co.uk
He claims it's not vandalism and his fellow 'Yellowists' say that he simply 'signed' the painting to "show that this painting is a potential piece of Yellowism." - thisisyellowism.com
There's an established tradition of artists (& non-artists) making changes to existing artworks. Can this 'vandalism' ever be acceptable?