Anyone with any version Word can download a patch which means they can open .docx files, and although it is an unbelievably crap format in lots of ways, I think it's pretty useless of any business which handles Word files day in day out not to be up to speed to that extent - Word 2007 also defaults to .docx, so it's not exactly new.
You can "save as" .rtf, or ordinary .doc, which is the easiest way.
(Word 2007 chews up .rtf files, in my experience, too - my OU students are allowed to submit it in, and I never know whether to take marks off for duff presentation or not. So they're not a panacea either.)
It's enough to make you go back to typewriter and postage stamps.
Emma
<Added>Naomi, sorry, realise that reads as if I'm saying you're useless - I was meaning magazines and publishers and so on, of course, not real people.