This is not something to be taken lightly. For the casual observer, her tactics might have given them a slanted and preconceived outcome based on her partiality to Joe Biden.
From the Weekly Standard:
White House records reveal that the moderator of last night's vice presidential debate, Martha Raddatz, visited Vice President Joe Biden at his official residence on March 26, 2012. Raddatz is an employee of ABC News.
As the records show, that day Raddatz visited the VPR (or, the vice president's residence) for a "Women's History Month Reception." That record was released on June 29, 2012.
According to Biden's schedule released the day before the event, "the Vice President and Dr. Jill Biden will host a reception in honor of Women’s History Month at the Naval Observatory."
There appears to be no pool report from the event, which presumably was widely attended, that might have recorded the details from the events.
On at least one other occasion, Raddatz visited the White House. According to records, that visit was on December 18, 2009. The reason for that visit is not know, though her meeting appears to have taken place in the Old Executive Office Building, and not the West Wing.
Before last night's debate, Raddatz came under scrutiny after the Daily Caller revealed she had attended President Barack Obama's wedding in 1991 with her now ex-husband, Obama administration official Julius Genachowski.