The whole idea of Google Custom Domains for Blogger blogs is great, as it gives us most of the benefits of an externally published blog, plus the New Blogger Layouts Templates features. But there's a major flaw currently in there.
If your blog is accessed as both "www.myblog.com" and "myblog.com", this won't work. AdSense appears to be sensitive to this problem. Tell Blogger that this is a major problem.