Fans saw it coming; a few new achievements that came out recently indicate that Nightwing is the fourth playable character. Considering the way that Joker worked in the last game, we expect Nightwing to be playable in "bundle packed campaigns, and ranked maps". We presume this means the character can be played in ranked challenge maps as well as the newly introduced campaign challenge maps.
It should be noted that you can only play Batman and Catwoman during the normal campaign, Robin appearing as a pre-order exclusive (Best Buy, GAME and Gamestation) and will be available via DLC at a later date.
In addition it is reported that Catwoman's role in Arkham City is exclusive to an online pass, forcing users to purchase the game 1st hand or paying for the pass separately via PSN or Xbox LIVE. This has yet to be officially confirmed though, but it does sound reasonable.
Arkham City sounds like what I wanted of Arkham Asylum: MORE in any way possible.
Batman Arkham City releases on October 18th, 2011 on the PC, OnLive, PlayStation 3, Xbox 360 and Wii U.