The Washington Post is reporting tonight that Senator John Kerry is under consideration for Secretary of Defense under Obama's second term, rather than State, which is the usual rumor.
If true, it could mean a special election later in 2013. It is widely assumed that a new Secretary of State will be named soon, with Hillary Clinton itching to go home. There's less rush making a change at Defense. So, maybe that happens in the summer, putting the special election in the fall.
The big question everyone has is whether Scott Brown would run. Note that the winner of the special election would need to run again in Kerry's regular re-election year of 2014 -- for Brown, that would be four full-scale, state-wide elections in less than five years. That's pretty daunting even for someone as indefatiguably a campaigner as he is.
Anyway, at this point it's just speculation. But we'll be keeping an eye on it.