Thanks UKHeliBob & Robin2... your comments explain perfectly why I am seeing these errors.
It sounds like AltSoftSerial might be worth exploring. Ultimately, I'd like to use this in my ATmega1284P Bobuino (I know... this chip has 2 hardware serial ports, but I am already using them for something else). Does anyone know if I can use AltSoftSerial with the 1284 so that I could create a "virtual hardware" port for the GPS, and use one softwareserial port for the OpenLog?