Marvel revealed that Sam Humphries Ultimates series will see Captain America become President of the United States in next week's issue #16. Like killing Peter Parker, this is the type of event that couldn't happen in any mainstream continuity (also, Superman couldn't be POTUS. See: Birth Certificate). In Ultimates #9, a human bomb exploded on the floor of Congress, destroying most of Washington DC. Meanwhile, anti-mutant sentinels and militias tore the Southwest apart and the West Coast formed an independent state, leaving the new President's work cut out for him. Comic Book Resources has a preview of the interior pages, and I love the press is calling him President Cap. Of course this raises a lot of questions. Will he keep Carol Danvers as Chief of Staff? Will Tony Stark finally get to be Secretary of Defense? Anyway, if you want to catch up on the newsest run of the Ultimates, the first two volumes are out in trades: Volume 1 and Volume 2.
Update: The Washington Post has an article worth checking out.