Well, the latest hearsay is in the ongoing WebmasterWorld thread where someone spoke to an official AdSense representative at Google, who said that if US publishers do not comply, Google doesn't have plans to ban the account.
Here is what the publisher said in the thread:
My friend attended PubCon and spoke to the Adsense speaker one on one. He was told, from what I was told, that Adsense was leaving it up to the publishers on how they wanted to handle the EU Cookie law and they would NOT be banning accounts of users who fail to meet the EU directive.
This was specific to US publishers, it is unclear how this impacts EU publishers. It is also unclear how legit this statement is.
Forum discussion at WebmasterWorld.