Check the campaign blog for the announcement.

Update: It's recent state Democratic chair and party insider George Jepsen:

“It begins with the war, but is not limited to the war,” Jepsen said. “I sincerely like Joe Lieberman. But I disagree profoundly with his unyielding support for George Bush’s war in Iraq, his embrace of the so-called ‘culture of life’ in overriding the rights of Terry Schiavo, his failure to be an advocate for civil rights issues, his support for school vouchers, his being the only New England Senator to vote to allow the federal government to put a natural gas pipeline in Long Island Sound.”...

Lamont’s presence on the ticket will help all Democrats, “because we will be speaking in one voice,” added Jepsen.

This is huge news, and has to be devastating to Lieberman. Establishment CT Democrats are feeling the breeze, and are jumping ship on Joe before he is able to jump ship on them.