iWork '08 supports OpenXML
As Ecma TC45 were working on what would become Ecma-376 there were changes to the specification. Microsoft hadn't released Office 2007, and so, as TC45 made their changes, the product team were implementing them. This meant that there was some change to the file formats from build to build of pre-release versions of Office 2007.
Why do I tell you this? Well Apple recently released iWork '08, which Brian noted provides high fidelity OpenXML readers. All goodness. Except when Bruce McKenzie tried to use it he wasn't able to open some sample OpenXML files provided on OpenXMLDeveloper.org.
I tried the same sample file in Word 2007.

Hmmm, sure enough the files were posted some time ago
I've sent a note to Doug to see if we can't get the sample files brought up to date - thanks Bruce.
Here's one of the .docx files from Doug's OpenXML Developer Workshop content opened in Pages '08 and Word 2007 below
and here's a spreadsheet made in Gnumeric, and viewed in Numbers with Excel nowhere in sight.
