Because of changes Microsoft has made in its recent Word release that are incompatible with our internal workflow, which was built around previous versions of the software, Science cannot at present accept any files in the new .docx format produced through Microsoft Word 2007, either for initial submission or for revision. Users of this release of Word should convert these files to a format compatible with Word 2003 or Word for Macintosh 2004 (or, for initial submission, to a PDF file) before submitting to Science.
(emphasis in original)
It gets even worse: Microsoft apparently did not make the new Equation Editor backwards compatible:
Users of Word 2007 should also be aware that equations created with the default equation editor included in Microsoft Word 2007 will be unacceptable in revision, even if the file is converted to a format compatible with earlier versions of Word; this is because conversion will render equations as graphics and prevent electronic printing of equations.
Nature, another journal similar in range and prestige to Science, has the same problem:
We currently cannot accept files saved in Microsoft Office 2007 formats. Equations and special characters (for example, Greek letters) cannot be edited and are incompatible with Nature's own editing and typesetting programs.
Never get involved in a land war in Asia, never agree to a battle of wits in which iocane powder is a factor, and never write papers for publication in Microsoft Word.
(h/t commenter james at Brad DeLong's blog)