When will Scott Brown take his seat in the U.S. Senate?
The question of timing seems less important this morning than it did before the special election, because, as I noted here, congressional Democrats appear to have realized that any attempt to try to complete congressional action on a health care bill before Brown takes office, would carry huge political risks for them, leading up to next November's mid-term elections. Such an attempt would have involved some combination of speeding up the legislative process and/or delaying Brown's entry into the Senate.
The Boston Globe reports that "Secretary of State William F. Galvin said he will send a letter with the unofficial results today to the secretary of the Senate, who has the authority to decide when to swear in the winner." That is standard procedure.
The 17th Amendment to the Constitution, which provides for election of senators (replacing the original method of appointment by state legislatures), allows a state to empower its governor to "make temporary appointments until the people fill the vacancies by election". That has been interpreted to mean that the term of an interim senator, such as Massachusetts' Paul Kirk, ends immediately after an election is held. The only exception is when it's the regularly-scheduled election for the next six-year term, in which case the appointment is valid until Congress reconvenes in January. To do otherwise would mean that the new senator would have been elected for a term longer than six years, which would violate the Constitution. That, of course, is not the case with yesterday's Massachusetts election.
Taking a historical example (not quite at random), Ted Kennedy won a special election for this Senate seat on November 6, 1962, and he was sworn in to the Senate the following day.