It seems that whenever I open the serial monitor in the arduino software, I get an error message saying that the serial port has been unplugged and that some data may have been lost. Weird thing is, I haven't unplugged anything. Anybody got any ideas?
What operating system are you on?
When you open the serial monitor on Mac or Linux (and maybe Vista), the board resets. That's probably triggering the message you're getting.
I am using a Mac... but this only occurs when I am using 4.8 volt hobby servos... other simple serial communication sketches don't seem to have this problem.
Is your circuit powered via the USB port on your Mac or do you use an external power supply?
If you are powering your circuit from the USB port on your Mac, my guess is that your circuit is drawing too much current from the USB port, especially since it is driving servos. Simple serial comms circuits don't draw much power.
The servos might be drawing too much current (current limiting) which causes the USB voltage to drop, which can cause the USB interface to drop out, hence your error message.
If you can, try and power your circuit from an external power supply with a decent current rating and see if that helps.
I tried powering the diecimilla through USB and the motorshield with a 9VDC wall wart and got the same result... I am still clueless
The motors are causing interference to the USB to serial bridge. Fit decoupling capacitors across the supply to the motors. Try 0.1uF on each motor to start with.