There's no release date announced for the PC version of the DLC, but it will be coming to consoles May 29. If the release of Arkham City itself is any indication, I'd expect it to be coming about a month later. Regardless, it'll probably be worth the wait, as Robin will be a playable character and Harley Quinn looks really bummed out. Also, beating the hell out of goons with that stick looks seriously fun.
Source: Gamespy