A WebmasterWorld thread has a lot of discussion around the AdSense code not complying with the W3C validation requirements. Some webmasters want their pages to be 100% validate, and adding this code may ruin that.
I am told the piece that invalidates the HTML is:
There is no word from a Google representative on this issue, yet. But I suspect we should hear soon about this. I mean, the thread is pretty active and it should be an easy fix for Google, I would suspect.
Forum discussion at WebmasterWorld.