Earlier this week we learned a major announcement from Activision would be made on Saturday. There was some speculation that this would be about a new CoD game and it looks like the rumours were right.
Activision send out an e-mail today that Call of Duty 4: Modern Warfare will be unveiled on April 28th.
We don't know anything about this game yet but I suppose that this game won't be a World War II shooter anymore. The "Modern Warfare" suffix seems to suggest that this game is either based on present day conflicts (like the war in Iraq) or perhaps on a fictional conflict in the near-future.
The other CoD games, like Call of Duty 3, were all based on battles from WWII. Personally I think it's good that they're trying something new as we've really been flooded by games based on WWII.