This time, it's Marion Berry of Arkansas (not to be confused with the infamous DC mayor). Berry has never had a tough re-election in his six terms in office, but his district pulled overwhelmingly for McCain in 2008. The Congressman cited health concerns as the reason for his departure.
The announcement is a bit of a sucker punch for Democrats. Berry had been an outspoken critic of Scott Brown's election in Massachusetts, calling Brown's win "shocking to say the least."