According to fans on the GTA Forums the latest patch (v.188.8.131.52) not only fixes some bugs but also includes achievements for The Lost & Damned.
Microsoft paid Rockstar, the guys behind GTA, around $50 million for the exclusive 360 content. However, releasing the content on PC would still allow Microsoft to ignore rivals Sony and the PS3.
Currently, there has been no official word from either Microsoft or Rockstar.
(Originally written for Game-Debate)