It violates the terms of use of Google AdSense program if you try to manipulate their Adsense scripts embedded in your site. However, there is an official guide from Google telling you how to do the split testing without violating their rules. You can view it here: http://adsense.blogspot.com/2006/08/abcs-of-ab-testing.html
Bear in mind the three fundamental principles of marketing: test, test and test
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