I was playing around with Office 2007 B1 over the last 2-3 days. It is well documented that the new .x formats (docx, xlsx etc) are compressed XML formats. It is not very well documented on how you can actually take a look into them easily. The easiest way, I found, was to just rename the file as .zip, open it and extract out the contents. You now have all the internal XML data as seperate files.
As of now, I am unable to do the reverse, of taking the extracted file, changing it and putting it back into a valid office document. Maybe it is to do with the fact that I am using winrar to create my zip files and not winzip. Anyway, will play around some more and let you know the result.