Photokina 2010 will be help Tuesday, September 21 to Sunday, September 26, 2010 in Cologne, Germany. That is a world stage,and one which Nikon has used many times over the past decades to premier big, new products that they are proud of...Photokina gets a huge amount of European press coverage...
I think better video is the area they will focus on in a D90 follow-up. It might be as simple as iterating the D90 to an s version called D90s, as KmH speculates above. It's always hard to predict what,exactly, Nikon will do, but the 2-year upgrade cycle means a D90 replacement or update/refresh/iteration could come at ANY TIME now...