According to this report Obama called McCain, “John” to needle him into losing his temper. It didn’t work and it just made Obama look disrespectful and arrogant:

Sen. Barack Obama and his debate prep team were surprised that Sen. John McCain remained calm and collected during the debate. Obama’s continual references to “John,” while McCain referred to Obama as “Senator,” was intended to needle McCain’s patience in hopes for a flash of McCain infamous temper. But McCain was, according to campaign advisers, ready for the needle and didn’t take the bait.

Who does Obama think he’s dealing with? McCain’s been through a lot tougher opponents than Obama and has had to deal with a lot more than Obama can throw at him.