According to TV Guide, Allison Janney will guest star on the episode to air Monday, September 5, at 10:30 PM ET on Showtime.
You can watch part of her appearance here. This link will probably not work for readers outside the U.S. People unable to access the site can try here, but they still may not be able to watch the video.
There is a new alternative for viewers outside the US. Go to MSNBC, and click anywhere where it says video. Then choose the Entertainment tab on the top, and the Showtime tab on the left and the clip should be available. Thanks to Stormeynight at TWOP for the suggestion. You will need Internet Explorer and Windows Media.