The Office 2007 file formats are open and well documented. Roughly speaking, all of the new file formats ending in "x" are zip compressed XML documents. For example:
To open a Word 2007 XML file Create a
temporary folder in which to store the
file and its parts.
Save a Word 2007 document, containing
text, pictures, and other elements, as
a .docx file.
Add a .zip extension to the end of the
file name.
Double-click the file. It will open in
the ZIP application. You can see the
parts that comprise the file.
Extract the parts to the folder that
you created previously.
The other file formats are roughly similar. I don't know of any open source libraries for interacting with them as yet - but depending on your exact requirements, it doesn't look too difficult to read and write simple documents. Certainly it should be a lot easier than with the older formats.
If you need to read the older formats, OpenOffice has an API and can read and write Office 2003 and older documents with more or less success.