From TV Guide's Ask Ausiello:
"Question: I heard Mary-Louise Parker was scheduled to return to The West Wing this season, but I figure she's got her hands full with Weeds. Do you know what the story is? — Chris
Ausiello: WW producer Eli Attie says Mary-Louise is "currently not in any episodes, but there are many surprises to come this season!" That Eli is such a little tease."
From an assistant to Mary Louise Parker via a related Yahoo Group regarding "West Wing":
"FYI: they completely directly misquoted her in that last article
that you sent me today!"
From the Toronto Star: "I would have continued doing that show, but when I got pregnant ... at a certain point, I couldn't, like, stand behind a desk anymore, or hold a file ... It just got to be ludicrous. So I had to leave.
'I loved that show. I was really happy on The West Wing. It was one of the best experiences I've ever had. I would go back any time — and, in fact, I am coming back to do a few episodes this season.
'It just never worked out that I was available to do it, you know, over a really long stretch of time. It's an hour show, and a 22-episode season, which is kind of brutal ... '
'I don't think I have that kind of muscle. I do admire it, though. It's a very particular skill. It takes tremendous concentration.
'It's a particular craft, and I don't have it. That's not my thing.'"
This is no confirmation that she will in fact appear on the show. Earlier this summer a person with contacts to Mary-Louise Parker posted the following at a message group dedicated to the actress:
"She is hoping to....If all goes well with scheduling she will..."
At the moment I would classify this as uncomfirmed plans.