I pulled the voltage regulator and found one clean brush and one carbon lined brush. Photos are available at photos.yahoo.com/tabijan in the MB Voltage Regulator album. Sorry about the quality, my camera doesn't take good close ups. I presume the problem stems from one brush not making contact with the armature (or whatever it contacts). I guess we'll find out when the replacement regulator gets here.
For the purpose of the archives, removing the regulator requires nothing more than a Philips screwdriver. I had to get to the alternator from below. Let's see how much fun it is to install the replacement regulator.
Thanks for all who offered help. It's all much appreciated.