This is a serious-ass problem with MSWord. You can't actually delete or clean out the metadata, either, so far as we've been able to figure out (there are scrubber programs that claim to do it, but they apparently don't actually work).
It's not just information about the doc you are sending, either - if it was copied (or sections of it were copied) from another doc, it will probably contain the meta-data of the other doc as well. So you also need to worry about Word docs you send to your own clients, because you're potentially divulging confidential information of the other clients from whose docs you copied text.
Seriously, we should all go back to WordPerfect 5.1.
