I have been receiving notices that Google Tags has been retired for all users as of April 29, 2011. Data will be available in your Google Places accounts until the end of May 2011 so make sure you record what you need to. Billing history will be available until July 31st, 2011.
As far as the reason goes, I am unsure. Seems that they have replaced it with Google Boost. You can call 877-503-0841 Monday to Friday 8am - 6pm to redeem your credit.