NBC released this information as a part of their May Sweeps announcement...
"On Thursday, May 15th back-to-back hour finales of "My Name is Earl," guest-starring Milano, and "The Office," featuring Oscar-nominated guest star [Amy]Ryan, will wrap up the season for those acclaimed comedies from 8-9 p.m. ET and 9-10 p.m. ET respectively."
Amy Ryan has been in "Gone Baby Gone" (Oscar nomination), was a regular on "The Wire" and was in "Dan in Real Life" with Steve Carell.
Update: New pic from NBC.
WHO could Ryan be playing? Oh the possibilites!! Head over to COMMENTS and let's discuss! (Warning: I would assume possible spoilers will be discussed - don't go into comments unless you want to know.)