The beta code for OpenOffice 3.0 has been released, and it includes Open XML read support:
Behind the scenes, OpenOffice.org 3.0 will support the upcoming OpenDocument Format (ODF) 1.2 standard, and is capable of opening files created with MS-Office 2007 or MS-Office 2008 for Mac OS X (.docx, .xlsx, .pptx, etc.). This is in addition to read and write support for the MS-Office binary file formats (.doc, .xls, .ppt, etc.).
Full text of the announcement: http://marketing.openoffice.org/3.0/announcementbeta.html