One of my sessions at the Ready summit is about how developers can take advantage of the new XML-based file formats for Word, Excel and PowerPoint. One of the really cool things is that your users don't even need to be running Office 2007 to read or write this format - they can be using anything back to Office 2000!
Grab the Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint 2007 File Formats and away you go. More details on using the pack are available here. Note that the pack even works for Word, Excel and PowerPoint 2003 viewers!