He continues to double-cross the party that took him back after he campaigned for John McCain, and also allowed him to hold onto his choice committee chair (Homeland Security).  He double-crossed the Democratic party again today by threatening to join the Repugnicans in a filibuster of health care reform if it includes the public option.  W T F.  

The Huffington Post has an excellent slideshow detailing the times he has stabbed Democrats in the back, in order to further his own goals.
Top 15 Lieberman Betrayals: Joe's Worst Double-Crosses