We'll hear the obvious names such as Bill Richardson, Wesley Clark, Joe Biden and, of course, Hillary Clinton. But in the end, I'm pretty confident that Obama goes with Virginia Senator Jim Webb. Webb is a decorated war-veteran, a former Secretary of the Navy and is widely considered tough and "patriotic". He could help out with any claims that Obama is soft, while also adding the military nod that McCain will likely speak of every other second. Webb could also cater to the war-loving right, given that his son is currently deployed in Iraq.
If not Webb, my backup would be Biden.
Cabinet Predictions: Secretary of State: Joe Biden Attorney General: John Edwards Secretary of Defense: Wesley Clark Homeland Security: Chuck Hagel