looks like what i was going to do originally would likely be better - namely take the 12 volts to the shield and use an lm7805 to get the local 5 volts for the max232.
I'm going to locate a gps and an LSM 303 compass in a separate enclosure that will be located about 20 feet from the arduino. I imagined that running 5 volts out to them and expecting to get usable signals back was a poor idea, so I'll run 12 volts out, reduce it to 5 locally for the compass and gps and then route their signals back to the board. I was lucky enough to find some 7 conductor 18 gauge shielded stranded cable for this so hopefully voltage drops shouldn't be a problem.
thanks for your thoughts.
john