Microsoft Word 2007according to Microsoft lawyers. Dell, HP, and lots of other 'partners' would feel this in their wallets. Not to mention the consumer (who now can go and discover other means of writing a letter).
This is the responds to the court ruling that Microsoft offended a XML patent. The ruling states that Microsoft can't sell MS-Word in its current form, so that means MS Office without its flagship Word.
This case proves once again that software patents are killing business. Not that I favor Microsoft in this, but basically the software patents are nothing more than a (logical) way of solving problems (how to do this, how to do that, etc). In theory someone could have filed a patent on 'breathing' (a way to solve the possible problem of dying). But there's prior art on that one :).
Anyway, should the XML capable version of Word go away, you can always use OpenOffice to open those documents. In theory (!!!) that should work, but we all know about Microsoft and (open) standards.