See the post for screen shots
Here are a couple cool screenshots from Stephen McGibbon where he shows an OpenXML spreadsheet file created with Gnumeric (runs on Linux & Windows), and the file is then opened up in Numbers (Apple's new spreadsheet app for the Mac).
This is pretty cool as it shows how the Open XML formats allow you to communicate across multiple platforms without ever even using Microsoft technology. Of course we hope you still want to use Microsoft technology, but for reasons other than file formats (rich UI; powerful features; rich integration with backend data; etc.). By opening up the formats though, it means that those folks who choose to use our technology will be able to easily share their content with folks who make other choices; and that increase the value of Office for those customers.