The latest rumour is that Spader is in negotiations to join the cast:
http://insidetv.ew.com/2011/06/24/ja...der-the-office
However, it appears he'll initially be hired as the new Regional Manager in Scranton, but he will very quickly work his way into a promotion and replace Kathy Bates' character (since she doesn't have time to make her infrequent appearances now that her other show was picked up for next season).
Spader will then continue as a permanent cast member and foil to the new-new Regional Manager, rumoured to be Catherine Tate's crazy British woman character. This would accommodate her commitment to appear in a play in London's West End this summer, which means she won't be available to join the Office cast until the fall (production of the new season will likely begin in July or August).