The offificial Google AdSense blog is offering now a complete tutorial about A/B testing.
Google AdSense has also provided some tips to design and run an A/b test.

A/B testing is the test of two ad situations, according to Google.

Tips provided by Google AdSense blog include:

• One of the ad configurations should be the configuration currently displayed one.
• One only one change should be made to the ad configuration in each A/B experiment.
• For one A/B test, all configurations must be running simultaneously.

You can read more about it at:

http://adsense.blogspot.com/2006/08/abcs-of-ab-testing.html