Iran has been promising hits from US getting involved in helping Syrian rebels (Al Qaeda to boot). Good chance this is related since those kinds of bombs originate from Pakistan/India
"Although it is not known yet who is responsible for Mondays heinous bombings at the Boston Marathon that killed three and injured more than 100 people, a source within Iranian intelligence services told WND the Islamic regime was behind them and to look for trails through Sri Lanka and Bangladesh.
U.S. was warned of terror attacks
http://www.wnd.com/2013/04/u-s-was-warned-of-terror-attacks/
"Pressure Cooker with TATP?"
By Larry Johnson
"If these reports are correct (and that is the key assumption), then the devices used yesterday point to originating from Pakistan/India. This type of device has been tied to muslim extremists and Maoists."
http://www.noquarterusa.net/blog/75800/pressure-cooker-with-tatp/#more-75800