Gamerscore Blog (thanks Voodoo Extreme) explains the recent image of Gears of War in a Games for Windows display (story) as an error, rather than a sly confirmation of long-rumored plans for a PC edition of the Xbox 360 shooter:

Here's what happened: These images were created using draft templates for the Games for Windows box art and a variety of Microsoft Game Studios cover images. Gears of War was accidentally included by the artist as one of the MGS titles used as examples to show what our retail setup would look like. This DOES NOT indicate a confirmation that Gears of Wars will be a Games for Windows title -- it was a concept image created by a concept artist, nothing more. In fact, if you look closely you’ll also see that Age of Empires III and Rise of Legends are also branded Games for Windows, and this is not actually the case (Age of Empires III: The Warchiefs is a Games for Windows branded game, but not the original Age III).
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We're incredibly excited about the success of Gears of War on Xbox 360, but we have nothing further to announce about Gears of War, or any other platforms it might come to, at this time.