Powell's endorsement is also very good for Obama's military and foreign policy credentials. Anyone who was on the fence about that should be reassured now that a man who was instrumental in the last 20 years of US policy is on-board. My rule is if you have a military doctrine named after you, you're opinion on military issues is worth listening to.
There's also the race angle... the radical right trying to make this endorsement a racial issue are making themselves look desperate, out of touch, and pathetic. There's plenty of good reasons for Republicans to support Obama. It doesn't have to be a racial issue. Anyone who tries to make it one is showing how brainwashed they truly are. There's quite a collection of white Republicans endorsing Obama too -- including William F Buckley's son. If you've got white people from the National Review supporting you I think it's safe to say it's not a racial issue.
From a media stand point Powell will be the dominate story tomorrow, perhaps into mid-week depending how far the Republicans want to go with the race baiting. If they really want to go down that road there will be a media backlash over it. That will give this story legs all week long.