There's no title yet and the plot is still being worked on, but they're hoping to air a pilot as early as May, according to the Wall Street Journal (the article is truncated unless you're a subscriber, but you can get the whole story at Grey's Anatomy Insider).
Although (Grey's creator Shonda) Rhimes and ABC think Walsh’s character can continue to evolve on Grey’s Anatomy, it became clear in recent months that her position on the show was changing, say people familiar with the matter. Now divorced from Derek and friendly with Meredith, Addison has had fewer ways to keep busy - and fewer fireworks to entertain viewers.
ABC is approaching the spinoff in a smart way, says Norman Lear, the TV producer who refined the art of the spinoff in the 1970s when his All in the Family spawned a record five new shows.
I've grown to love Addison since she first appeared on Grey's. She was introduced as something of a witch, but the writers promised that it would become impossible to hate her, and they were right. Her desperate vulnerability in trying to win back the husband she'd cheated on won me over. As much as I enjoy the heat between her and Alex on the show, if they're struggling to find stuff for her to do, spinning her off into her own deal is great news.