Solution
You can use a lower level serial VI called Close Serial Driver.vi.
In LabVIEW 5.x, this VI is found under
\Labview\vi.lib\Instr\_sersup.llb. In LabVIEW 6.0, it is located under
\LabVIEW 6\vi.lib\instr\Serial.llb.
If you include this VI in the application, it will allow other programs to use the serial port without having to exit LabVIEW. You can then re-access the port in LabVIEW by subsequently running
Serial Port Init.vi, and
Serial write.vi, or
Serial Read.vi.