Well, as Modern Warfare 2 Blog reports, it turns out that some intrepid Xbox 360 players have found a way to unlock one of these hidden modes, Global Thermonuclear War, and they were kind enough to create a few gameplay videos to prove it. Why, here's one now:
Microsoft announced during their CES 2010 keynote address that the first round of downloadable content for MW2 would be releasing exclusively on the 360 this spring. Could these hidden maps be part of that DLC? Doesn’t seem that hard to believe. I guess time will tell.
Source: Modern Warfare 2 Blog