Uh oh, this sounds really bad! LWN is reporting that:
Here is a press release from legal firm McKool Smith, which is quite proud at having gotten a US court to rule that Word violates patent #5,787,499. “Today’s permanent injunction prohibits Microsoft from selling or importing to the United States any Microsoft Word products that have the capability of opening .XML, .DOCX or DOCM files (XML files) containing custom XML.” The text of this patent is quite vague; if it stands it could almost certainly be used to make life difficult for free software as well.
Microsoft taking a beating for this is not something to celebrate, this is yet another example of how software patents are really bad for all the players in computing.