There was a recent announcement that Microsoft would publishing their Quarterly Worldwide Uptime - http://blogs.office.com/b/office365tech/archive/2013/08/08/cloud-services-you-can-trust-office-365-availability.aspx
The uptime is published here - http://trustoffice365.com/
I went to the web site today and now see that is being published. For instance today is 8/30/2013 and it stating 99.97%, well better than 99.9% financially backed SLA - http://www.microsoftvolumelicensing.com/DocumentSearch.aspx?Mode=3&DocumentTypeId=37. If you click on the Learn More, it will show you the last four quarters.
Recommend that you read the announcement for more information about it.