New XML File Formats for Office 12
Word, Excel and PowerPoint will get new default file formats based on XML. The extensions will change from .DOC, .XLS, .PPT to .DOCX, .XLSX, .PPTX.
The new file format is actually enclosed in a ZIP file. Change the extension name to .ZIP and you'll be able to double-click and get access to all the pieces of the new format. There will be updates for previous versions of Office to enable this file format to be used with those apps too.
For more info there is a video interview with Brian Jones on Channel9, Brian is blogging now, and there are a couple of White Papers on MSDN giving some technical details.
http://channel9.msdn.com/ShowPost.aspx?PostID=73329
http://download.microsoft.com/download/c/2/9/c2935f83-1a10-4e4a-a137-c1db829637f5/Office12FileFormatDevPreviewWP.doc
http://download.microsoft.com/download/c/2/9/c2935f83-1a10-4e4a-a137-c1db829637f5/Office12NewFileFormatsWP.doc