Interestingly, a number of folks have not only been able to download it but also play the new content. The forum member continues, writing, "Those who bought them right away are playing them RIGHT NOW -- if you attempt to download them now you will receive an error message ... those [of] you [who] received the first error before Microsoft was alerted can redownload them fine."
These statements appear to be confirmed by the flood of excited responses of people playing the game, as well as by statements from Epic's Community Manager "Flak," who explains, "This was a bug or mistake. The DLC is still scheduled for release on the 28th for 1,600 Microsoft points (All Fronts) and 1,200 Microsoft points (Dark Corners)." Unsurprisingly, Epic asks forums users not to upload any video or screenshots of the accidentally released DLC -- so, ya know, you'll probably be seeing it everywhere by noon today ...
Update: Microsoft has officially responded to the situation and released this statement:
"Earlier this morning, the "Gears of War 2: All Fronts Collection" Game Add-on was briefly made available on Xbox Live for 800 Microsoft Points. The content has since been removed and will launch on Tuesday, July 28th worldwide as planned, together with the "Dark Corners" Game Add-on. Pricing will remain as originally intended (1600 and 1200 Microsoft Points for "All Fronts Collection" and "Dark Corners," respectively). The small, lucky group of Xbox Live members who were able to download the content during the short window it was available will be able to retain the content for the price at which it was purchased as well as keep all related Achievements. There won't be any penalties for those who purchased this content early. Think of it as a lucky treat for those with quick reflexes!"
There you have it, folks! Enjoy!