Today ISO have approved ISO/IEC DIS 29500, Office Open XML File Formats as an International Standard by support of 86% of 87 countries during approval process. To gain more information about Open XMl you can visit the sites below.

http://openxmldeveloper.org/default.aspx
http://www.openxmlcommunity.org/

Also you can reach ECMA's declaration from
http://www.ecma-international.org/news/TC45_current_work/ISO_and_IEC_approve_Office_Open_XML.htm

and Microsoft's press announcement from
http://www.microsoft.com/presspass/press/2008/apr08/04-01OpenXMLVotePR.mspx