The ability to save Xbox 360 game saves to the cloud is pretty cool. It means you can create virtual backups of all your save files, open them on another player's system, and never lose your progress if you had to replace your Xbox 360. Unfortunately, cloud stuff always has hiccups, and that's being demonstrated with the new release of Mass Effect 3.
The Mass Effect franchise follows a continuous storyline, and as a result the game developers have encouraged players to complete each title before moving on to the next one. Players who had a Mass Effect save file could transfer vital data to their Mass Effect 2 game, and now that Mass Effect 3 is out the same thing applies for ME2 to ME3. Unfortunately, though, Mass Effect 3 is completely unable to identify Mass Effect 2 save files if they were stored in the cloud.
"While Mass Effect 3 supports cloud saving for new or successfully imported Mass Effect 2 saves, it does not support the importation of endgame saves directly from the cloud. Additionally, this can cause it to not recognize save files that have been transferred via cloud to a console other than the one where they originated," publisher EA confirmed in a statement. If you're able to go back to the console in which you originally saved the game and create a local copy of the save file, you're good to go. But if you no longer have access to that system, you are screwed. EA has given no indication that this problem is fixable.