Joystiq has a graph from Warcraft Realms showing declining player activity in World of Warcraft since February. Now we can discuss endlessly how exact these sort of data are. But I think anyone is aware that the "we now have 9 million players" press release from Blizzard never came, so some decline is probable. Note how on the graph the player activity was going up every month, peaked when The Burning Crusade came out and is going down since then.
I think that some top managers and game developers at Blizzard should think hard what exactly they did wrong with The Burning Crusade. If you make an expansion which throws your subscriber number development into reverse gear, you shouldn't plan to make the next expansion in the same way.