This is not your average MBTA bus! Rather than being wrapped in an ad, this bus is wrapped in a moving memorial, created by Thomas Starr, a professor at Northeastern University. The memorial consists of quotes from family members of children that were murdered, as well as their year of birth and death. The bus has been in service since late October 2006, and will hopefully be around for a long time to come. It's very eye catching - the color of a school bus, and covered in statements like "He liked chocolate", which deepen the sense of loss.
You can read more about the motivation behind the project here, and an article from the BU daily student newspaper here.