The revolt of the generals
Ruchira Paul has a good, concise overview of the generals' revolt (now including Wesley Clark).
Rootless Cosmopolitan speculates about the reasons for doing it now and why Rumsfeld is the target:
Rumsfeld is, in some ways, low hanging fruit for the generals. After all, he's the civilian political appointee who translates administration policy into the military, and as such is the obvious target of a backlash by the uniformed professional military against the administration. If the generals were going on Sunday talk shows calling for President Bush to resign, they'd be deemed to be part of a coup. The generals' grievances over Iraq, and the no-win situation in which it has placed the U.S. military (and the epic weakening of the U.S. strategic position more generally it has occasioned) obviously extends to President Bush, Vice President Cheney and others. But to avoid appearing insubordinate, the generals are couching their criticism in terms of policy choices made in the Pentagon, their immediate overseers. (In corporate culture, disgruntled employees are permitted to complain to Human Resources about their immediate managers, but nobody in the company is going to hear out any complaints they may have about the strategic choices made by the CEO -- thus the generals targeting Rumsfeld, rather than Bush.)
There's no obvious reason by the logic of the current situation in Iraq, or decisions that may be made shortly, for the generals to choose this moment to launch their offensive. They all believe that the U.S. needs to remain in Iraq as long as it takes to stabilize it in some way (although they may well differ with the administration on what that might involve).
But given what Seymour Hersh's sources in the military and intelligence communities are telling him about plans for military action against Iran, there's certainly a clear motive for those seeking to save the U.S. military from further calamitous misadventures to pick a very public battle with the administration over its handling of strategic matters.
***So why go after Rummy if the goal is to stop another bout of reckless adventurism for which the men and women in uniform pay the price? Well, it's a key battle in pursuit of that goal, because by publicly challenging Rummy's handling of Iraq, the generals send a none-too-subtle signal to the U.S. public, in an election year, that the Bush administration is strategically incompetent. And that would make it harder for Messrs. Cheney and Rumsfeld and co. to open a second front in Iran.