Just got a notice from WB Games PR:
Warner Bros. Interactive Entertainment and Rocksteady Studios are releasing the PC version of Batman: Arkham City in November. The Xbox 360 and PS3 versions of the game are confirmed to be final and will be available beginning October 18 in North America, October 19 in Australia, and October 21 in Europe and other global territories.
If you recall, Batman Arkham Asylum was delayed on PC when it first came out for some enhancements and features, such as PhysX support for nVidia cards. It seems that may be the case here. No official reason as to why has been stated.