Tomorrow we'll be seeing maintenance for all realms between 3:00 a.m. PST / 6:00 a.m. EST and 11:00 a.m. PST. / 2:00 p.m. EST. Ideally these things don't go over, but as you know, they often do. Look to posts throughout the morning on WoW Insider for updates.
We know that there'll be a few bug fixes, which we posted about earlier. There is still no word if this will be a server side only fix or if there'll be a client side patch, presumably patch 3.0.9. Again, we'll get the news up on the site as soon as we find out.