So far, WBIE is announcing Robin will only be playable through a pre-order exclusive with Best Buy known as the "Tim Drake Robin Pack." The exclusive will be made available across all three versions of the game: PC, Xbox 360, and PS3. As the name of the pack suggests, the Robin in Batman: Arkham City will be the Tim Drake version of Robin and he'll have his own set of gadgets and moves. The Tim Drake Robin Pack will allow Robin to play across all challenge maps in the game as well as come with two additional challenge maps, the Black Mask Hideout and Freight Train Escape. Lastly, a bonus Red Robin character skin will be made available through the pack.
WBIE also released an image of what Robin will look like in Batman: Arkham City, and I must say he looks like a complete badass. It's a good thing he isn't available to play through the single player campaign as I think the Arkham City criminals would be more afraid to bump into Robin than they would Batman.
[Update: Shacknews has confirmed with a Warner Bros. representative that Robin will be a timed exclusive and will be made available sometime after the release of Batman: Arkham City. No word yet as to the expected price or date of release, but I'm sure many future Arkham City players are jumping for joy at the prospect of playing the Tim Drake Robin Pack]