Jon Cohn has details on the general's big announcement. (Video here.) Two things strike me: One is how pointed the endorsement was. This was not a mere nod to Obama but rather an indictment of John McCain, Sarah Palin, and the state of the GOP.
The second is about Powell himself. I wonder if he sees this in any way as a moment of both revenge and redemption. Powell trashed one of the most sterling brands in American politics with his pivotal 2003 UN speech touting evidence of Iraqi WMD that was later discredited. You can debate whether Powell should have known better, but it's clear that he feels resentful towards the Bush administration. Breaking with the GOP may have been, in part, a way of paying back the neocons who sucked him into a conflict that defied the core principles of the military doctrine that bears Powell's name.