Waste containers as bomb receptacles
It's far too soon to draw conclusions from what happened in Boston yesterday, other than recognizing that we are going to have to significantly extend and expand our protocols for event planning to be very robust with regard to security and safety matters.
There are people who do take advantage of events and festivals to get their message out.
For example, the Zendik commune (They sell "Stop bitching, start a revolution" bumper stickers, see this article from the Washington City Paper) come out to street festivals to sell their wares, but on the street--not paying for a booth. Other people and organizations are known for doing this.
When I was a Main Street manager we organized an event that got some advance press in the Washington Post, and clearly the person who operates this protest vehicle (often it is parked on Capitol Hill) saw the piece and figured if he came to Brookland during our event he could reach new audiences.
Transportation systems and festivals and special events (e.g., the bombing at the Atlanta Olympics) are great opportunities for mischief and mayhem.
-- Planning And Managing Security For Major Special Events: Guidelines for Law Enforcement, US Dept. of Justice
-- Event Security Planning Tool, Rural Domestic Preparedness Consortium
-- Special issue, Special Event Security: Defending Against CBRNE, Domestic Preparedness Journal
-- National Special Security Events: Transportation Planning for Planned Special Events, Federal Highway Administration
And it does put into perspective increasing distaste on the part of cities for closing off major streets for special events like big street festivals, because it stresses other elements of the mobility network.