The West Wing News Blog stresses that so far the information below is a rumor. While the first concrete bit of information is based on the audition sides which are available to anyone (see previous post), the other information is so far unsubstantiated and could be speculation based on guessing and other rumors.
From TV Guide
Question: Please give me some scoop on The West Wing. — Emily
Ausiello: We're going to see Toby (Richard Schiff) again right before the big election — on Halloween, to be exact — when he returns home to take his son, Huck, trick-or-treating. Oh, and the show isn't returning next year. Nothing official from NBC yet, but reliable sources tell me it's all over in May.