Okay, I managed to fix the problem. This morning I learned that other fonts in Word were displaying just fine, except for the ONE font that I use the most (Brush Script MT). Font Book showed no problem with the font.
I did some looking around at similar issues on
http://word.mvps.org/Mac (Word:Mac), and found a posting suggesting trying an application called "Linotype FontExplorer X". After installing that app and running it, I noticed that the Brush Script font was appearing twice in its library. I deactivated one of them (the app said it was the one located in my user library), and now Word is displaying the font just fine.
I'm still not exactly sure what caused the problem to begin with (argh), but at least it works now! Thanks much for all your help, Dave.