Hmm...it appears that MA does have runoffs for primaries; I do find any provision for a runoff in the general election. Indeed, it looks like Romney got only 49.77% of the total vote, even if it was 51% of the enrolled party vote.
Republican Mitt Romney 1,091,988 49.77
Democratic Shannon O’Brien 985,981 44.94
Green-Rainbow Jill Stein 76,530 3.49
Libertarian Carla Howell 23,044 1.05
Independent Barbara C. Johnson 15,335 0.70
Write-in All others 1,301 0.06
I meant I do not find any provision for a general election runoff.
Primary runoffs are only in TX, OK, AR, LA (called general election), MS, AL, GA, and SC, plus NC if no one gets 40 percent. There have never been runoffs in party primaries outside of the South.