From CNN:

Massachusetts Gov. Deval Patrick announced he has chosen longtime Kennedy friend Paul Kirk to fill the vacant seat left open by the late Sen. Ted Kennedy until a special election is held in January.
Patrick said Kirk will begin serving immediately.
“He’s a distinguished lawyer, volunteer and citizen and he shares the sense of service that so distinguished Sen. Ted Kennedy,” Patrick said. “Paul will not seek the open seat in the special election that’s coming up in January but for the next few months he’ll carry on the work and focus of Senator Kennedy, mindful of his mission and his values and his love of Massachusetts.
Kirk is a former chairman of the Democratic National Committee and currently heads the John F. Kennedy library in Boston.
“I was personally privileged to have had senator Kennedy’s friendship, his trust, and his confidence,” Kirk said. “He often said that representing the people of the Commonwealth of Massachusetts in the Senate of the United States was the highest honor that he could possibly imagine. And it certainly is nothing i imagined but it will be my highest honor as well.”
Patrick said Kirk was chosen among 6 finalists for the post.

Also: “Meet Senator Paul Kirk,” from NPR

Update:

President Obama responds to the pick, from the Washington Post:

President Obama today hailed the appointment of Paul Kirk as senator from Massachusetts.
“I am pleased that Massachusetts will have its full representation in the United States Senate in the coming months, as important issues such as health care, financial reform and energy will be debated,” he said.
“Paul Kirk is a distinguished leader, whose long collaboration with Senator Kennedy makes him an excellent, interim choice to carry on his work until the voters make their choice in January.”

Rob Jesmer, the executive director of the National Republican Senatorial Committee, responds:

The Democrats’ power play in Massachusetts has nothing to do with principle, and everything to do with politics. With their unpopular government-run health care bill on the brink of failure, Democrats in Washington desperately need another vote in the U.S. Senate, and it is clear that this Administration will stop at nothing to ram it through the Congress. By meddling in the affairs of state politics just weeks after promising to leave it up to the elected Massachusetts officials, Democrat leaders in Washington have demonstrated a willingness to put partisan politics over principle, a far cry from candidate Obama’s pledge to change the way business is done in Washington.