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Some important changes are coming to AdSense on Google Sites. By August 30, 2013, it will no longer be possible to make updates to AdSense on any of your Google Sites.
[edited by: martinibuster at 7:43 pm (utc) on Jul 25, 2013]
[edit reason] Fixed link and added explanatory quote. [/edit]
But more likely it's more trouble than it's worth. Google's doing a lot of that lately. Wouldn't surprise me at all if they dumped Sites, or rolled it into Blogger.