Hi Roger:
"Maybe"

Generally, "No". But PC Word is capable of creating content
that Mac Word can't handle (and vice-versa). You will have to suck it and
see. The problems occur mainly in graphics, so if the material does not
contain graphics, you should have no trouble.
The answer to that question is "Yes, if the program that opens XML is
Microsoft Word. Otherwise, "probably not"."
To explain: Mark-Up language documents are not complete in themselves. By
"mark-up language" I mean Standardized Generalized Mark-up Language (SGML)
and any of its derivatives such as HTML and XML.
Each of these file types requires a Document Type Definition to say which
tags are legal within the file, and a Formatting Output Specification
Instance, to tell the recipient application how to handle the tags.
In XML, you can combine these two external files into a single XSLT (XML
Style Sheet and Language Transform).
Microsoft Word has the Microsoft Office Open XML (OOXML) XSLT built-in. The
style sheet for Open Document Format (ODF) can be added.
But the question you really want the answer to is "Can I use an XML-capable
application to open Word .docx documents, and expect to see everything Word
can create?" The answer to that is "Theoretically, yes, but in practice,
FORGET IT!!"
Creating an XSLT of sufficient complexity to render everything Word can
throw at it took Microsoft about four years. A smaller development
organisation should allow ten years to create one. And Microsoft is not in
a hurry to sell theirs
Now: If all you wanted to know is "Can Mac Word 2004 open and edit Word
2007 .docx files?" the answer is again "maybe". Again: Word 2007 can
create things (mainly: graphics) that Word 2004 cannot edit. It will
normally be able to "display" them. But it can't edit them.
You will encounter very few of these: most users have no idea what they are
or how to create them, on the PC side. But it is theoretically possible
that you will find objects in a document that you will be able to display
but not edit properly.
It is extremely unlikely that you will encounter anything created by Word
2007 that you cannot display in Word 2004. But it remains likely that you
will find objects in a Word 2007 file that were NOT created by Word 2007.
Those: you're on your own.
However, your editor should be able to edit very happily in Word 2004. An
editor almost never wants to make changes to the "pictures", and he or she
will be able to edit everything else that could be in the file.
You, on the other hand, need to make sure that they install the RELEASED
version of the OOXML converter from the Microsoft MacTopia website.
Otherwise, Word 2004 can't read or save .docx at all. The released version
will probably be posted tomorrow or the next day (right after Office 2008
ships...). There is a beta version there at the moment, which is good, but
not fully capable.
Hope this helps