Google is going to implement an improvement to the top ad placement formula at the end of August.

Currently Google looks at a multiplication of the quality score and the actual cost per click.
Google will carry on weighing the quality, BUT: the actual cost per click will be exchanged to the maximum cost per click.
Why is this?
The actual cost per click depends too much on other advertisers. There can be really good ads with a great quality score but they will not automatically appear in the top positions when they bid less than other advertisers.
The new formula will give you more control over the ad-ranking, but you might also end up paying the maximum cost per click you chose.