Internal Israeli politics, like a lot of things in Israel, changes from one day to the next. This morning both Galei Tzahal and Haaretz are reporting that Olmert has not rejected Avigdor Lieberman's party out-of-hand, and the Kadima party is making overtures to him.
This puts Amir Peretz in a difficult situation, because he came out with a statement before the election that he wouldn't sit with Lieberman. He supposedly reiterated this claim to Achmed Tibi as late as last night, according to this article in the Jerusalem Post.
Peretz's statement shows his inexperience in politics at this level. Closing out your options before you have to is never a good idea in this country.