Update 1: Just spoke with Pete Hines, and said,

It’s being uploaded now and should be available by this evening.

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The Pitt, the DLC that you skipped work to play, has been pulled off of Xbox Live due to problems with the english version. We spoke with Bethesda, and they responded with,

I just heard from Pete Hines, Product Manager for Fallout 3.  He said they are continuing to look into the issues people are experiencing with The Pitt for Xbox 360. As of now it looks like the file for the English version on Xbox Live was somehow corrupted. They are currently working with Microsoft to remove the existing file off of Live to prevent any further downloads. Their plan is to replace it with a new file as soon as possible. They hope to have another update shortly.

Thanks for your patience.

For those that purchased the DLC, you’ll be able to download the new version for free once it becomes available. We’ll be sure to let you all know with a new post when the content is up and ready to be played.