This is in some ways interesting, and in some ways to be expected.
It is interesting because you might have assumed that Powell, a military guy and a Republican who also happens to be a political donor to McCain -- might stick with him.
So why do this? You can certainly understand why Powell would be tempted to support the first African-American candidate with a legitimate shot at the presidency.
But aside from race, I think there is another similarity between Powell and Obama; the less you know about their political views, the more you like them. Frankly, had he wanted to, Powell could have probably been elected president as a Republican if he simply refused to say anything.
A cynical view for this move for this move, of course, is that Powell clearly cares about his legacy and this is his way, perhaps, of redeeming himself to the media and insiders.