The new adCenter release is currently planned for tomorrow - Saturday, 2/18/06. The update starts at 3 A.M. Pacific Standard Time (PST) and will take approximately 24 hours. During this time, you will not be able to sign in to adCenter, but your ads will continue to be served and be tracked for reporting.
So the new UI is being launched at the moment, but despite assurances that ads are being served, I have not had a single clickthrough since it started - and maybe not for a few hours prior to that as well. And doing a search at adcenter.search.msn.com shows no ads on any of the serp results either - for my keywords and for other popular PPC keywords as well.