The OpenXmlCommunity.org site has come online, listing supporters of Open XML. Brian Jones has the details here: http://blogs.msdn.com/brian_jones/archive/2007/05/08/openxml-community-growing.aspx
David Carlisle has a post on how Open XML equations and MathML equations can be converted to HTML: http://dpcarlisle.blogspot.com/2007/04/xhtml-and-mathml-from-openofficeorg-22.html. He also added some more information today at http://dpcarlisle.blogspot.com/2007/04/xhtml-and-mathml-from-office-20007.html.
Maarten Balliauw has a great new article on OpenXmlDeveloper.org that takes you through creating an Open XML spreadsheet with the PHPExcel API that he and Jakub Vrana have put together: OXD: http://openxmldeveloper.org/articles/1515.aspx. This sample uses formulas, images, and cell styling.