Hmmm, I'm not sure if Massachusetts law is any different, but from what we learned from the ordeal with Ron Blagojevich in Illinois after Obama left the Senate, it's typically the governor that appoints someone to replace the Senator until a special election. The choice has to be confirmed by the state Senate.
Matter of fact, I think this provision is in the Constitution, rather than state laws.
Although, I've got to say, I liked Blago's style. Right when the end was near he made sure he was a sufficient pain in the ass by spitefully nominating Roland Burris to the Senate in Obama's place. He knew the brouhaha over the state senate not confirming him because he was black would happen, and made sure that his nominee got in. Crafty, very crafty. Totally something I would do, by the way.