It all depends on context, really. Avoiding eye contact can be aggressive, or it can be defensive and it can be deferential.

In a shop the other lady the lady at the till was talking to a colleague for a longtime, while a queue built up. Naturally, neither of the two even glanced in the direction of the waiting customers or allowed any eye contact, until they were done.

In that context avoiding eye contact is obviously a way of putting up a barrier, putting the customers on hold so to speak, and it felt quite an aggressive thing to do.

In other settings it's more defensive - for example in a setting where eye contact is liable to be interpreted as making some kind of comment on a stranger's appearance or behaviour.

It isn't just humans that last applies to either. Generally speaking eye contact seems to be taken as a challenge or threat by many animals - you can get even bitten by a strange dog that way if you aren't careful.

But I still think in the context of that debate refusing eye contact would most likely have been intended by McCain's coach as a way of putting Obama ill at ease.