There's an update: http://blogs.msdn.com/b/windowsazure/archive/2012/03/09/summary-of-windows-azure-service-disruption-on-feb-29th-2012.aspx
The bug specifically was in generating certificates and setting the end date by taking the current date and adding one to the year. A classic mistake - we just had a TDWTF about this very error on, of course, 29 February - and the sort of thing that makes you wonder who they're letting write critical infrastructure code for their much-hyped PaaS cloud offering.
And, as pointed out by others previously in this thread, it would have been simpler to use their own framework date class, which does one-year-in-the-future trivially.