Word Doc Webbug Howto
And there you have, it. You now have a functioning webbug in your .doc file that doesn't spawn images or other things that would be more obvious to even the casual observer. The other nice thing is that style sheets don't give errors when they fail. Of course you can do the same thing with 1x1 pixel images using something like the following:
I like the CSS method better, however, because it is lesser known and Word is less likely to modify that section of code upon editing it, for now. Just remember, if you edit the file in Word, you need to verifiy that your change was not overwritten (this is prett unlikely, but check anyway). Happy bugging!