Several news says:
Shannen Doherty, is nearing a deal to return TEXT more episodes later this season. Executive producer Gabe Sachs would only confirm that she's "in talks," but my 90210 spy insists "she's coming back. The deal isn't officially done, but it's happening."
http://ausiellofiles.ew.com/2008/10/jason-priestley.html
http://omg.yahoo.com/news/jason-priestley-shannen-doherty-are-back-in-the-90210/13972
So she returns just for 2 episodes .... but I hope than she will do much much more