In an interview with Variety, Rockstar's main man Sam Houser, when asked about whether the extras would be landing in 2008, said, "“I’m not sure. It’s going well. But we don’t know a date. We hope to be announcing that in the next few weeks." Seeing as the next few weeks sees this year come to a close, you can safely assume you won't be doing an extra load of so–so missions until 2009.
Initially, the DLC had been slated to appear in the months after the game's April release, before it got shunted back indefinitely. It remains an Xbox 360 exclusive, but Sony bosses recently suggested it would be showing up on the PS3 too soon.
So, the wait continues. Looks like we'll have to rely on GTA Chinatown Wars for our Liberty City hit.
GTA IV DLC
On sale: TBA