This is fun
http://www.270towin.com/
Mess around with it!
Assuming everything goes right for McCain and he wins NH, VA, CO, FL, IA, MO and NM, (and also assuming he doesn't pick up the long shots of MN or WI), then he doesn't need OH, IN or PA. He would have exactly 270 electoral votes, exactly two more than Obama. Another way to look at that is that Obama needs OH, MI and PA very badly.
But how likely is that? Not very. So let's try the experiment the other way; assuming McCain can win PA, OH and MI, what does Obama need? He basically needs to win all but one of the other contenders (NH, VA, CO, FL, IA, MO and NM).
So this is why I think PA, OH and MI are the biggest battlegrounds to watch, even though the Obama campaign has to look everywhere, not just these three states. But I figure the other competitive states are likely to split rather than end up in the same candidate's column, while PA, OH and MI could go as a block.