One of the reasons many chose Grand Theft Auto IV on Xbox 360 over the PS3 and PC versions was because Microsoft had secured a deal with Rockstar Games to develop exclusive DLC for their shiny white box (or black for the Elite systems). However, the terms of the arrangement were never formally announced, leaving speculation on whether the DLC was a timed or permanent exclusive.
Well, a recent title update for the Games for Windows version of Grand Theft Auto IV has mysteriously added achievements for The Lost and Damned DLC. Rockstar has made no official announcement concerning the discovery, although it is possible Microsoft are allowing them to release the DLC for PC since it's running on the company's own platform.
For those who are curious, the update also gives players more freedom in customising controls, converts all region SKU's into the same SKU, and fixes up several multiplayer bugs.