Batman: Arkham City (despite carrying on the role in the first game). Instead, veteran voice actor Tara Strong will take her first stab at Harley.
Strong has been doing cartoon and video game voice overs since the late 80s. Some of her roles include X-23 in Marvel vs. Capcom 3, Rikku in Kingdom Hearts 2 and Sheegor in Psychonauts. She also voiced The Huntress in the Batman: The Brave and the Bold animated series.
A brief Q&A with Strong has been posted on the official Arkham City site, in which she says she wants to "honor" Harley's established voice -- but until we actually hear her rendition, we can't help but morn the loss of Sorkin.