Scott Brown will not be running for Senate in the special election.
My guess is that he decided that the Republican Party is just too damn radioactive in Massachusetts this year.
It looks like the Republicans may try to convince former Governor William Weld, the only Republican I ever voted for,* but I cannot imagine that the Club for Growth crowd would not end up subverting a campaign of his.
He is an old-line New England liberal Republican, and there are significant elements of the Republican base that would support a Democrat against him, just as the did with Lieberman over Weiker in the 1980s.
*He was running against BU President John Silber, who was a f%$#ing lunatic. Even though I still believe that it was the right thing, but I felt dirty afterward.