PROBLEM SOLVED ... or at least ARDUINO EXONERATED
This was a lesson in "never assume your power supply is good".
Something on the power bus is drawing the voltage down to about 2.1V. When I plug in the serial port, it is providing 5V to the Arduino, so everything works. Without the serial port, for whatever reason, my supply is only delivering 2V.
If I unplug the bus from the supply, it gives me 5.1V ... If I unplug the servo board and leave everything else connected, it still drops to 4.3V
Which in itself is a bit odd... this servo controller I'm debugging SHOULD be the only thing drawing more than processing power from the bus. I am really surprised that even the servo driver load would pull the supply so far out of regulation without shutting it down completely.
Anyway, the problem is clearly not with the Arduino itself, but rather with the power supply. I appreciate the debugging advice, your suggestions helped me pinpoint where the problem was coming from very quickly.
The process from here should be fairly straightforward... unplug ALL of the boards from the bus, check the voltage, then (assuming it is 5V unloaded) start adding boards back one at a time until I find either the offender or the limiting number of loads. It will be tedious, but it's a hobby. I've got nothing but time.
THANK YOU