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Possible uses: (1) serve different colors or sizes to reduce ad-blindness, (2) serve different channels, tracking anything knowable—hour of day, member status, (3) tie differences in aesthetics to channels, to track the most effective colors, sizes, etc.
I asked this question here once before, but got no replies. Google misunderstood my question and sent me a form letter about how to insert JS into webpages.
I don't see why you couldn't swap AdSense in and out using PHP or SSI, but I would be careful about constantly changing the ad format. You might conceivably get some bizarre side effects, such as excessive spidering, poor targeting, or frequent manual checks from Google staff.
Google, most esteemed AdsenseAdvisor,