Hey Mee_n_Mac and Chanler -
Many thanks to you both for your help! I walked through all your suggestions and got the results you suggested I should be getting. As I final step you suggested "check the voltage at both ends of the resistor and post back." I'm getting about 1.8v across the resister for about 1-1.5 secs, then flipping to -0.4v for 0.5 secs or so, constantly repeating. This is despite rock steady 4.89v across the rails. Not sure what this means.
FYI, I did flip the diode at one point just to see if it made a difference; did not.
One strange thing: The schematic that Chanler provided lists a 10k resistor, as does the schematic that came with my kit:
However, the schematic that Mee_n_Mac provided lists a 2.2k resistor, with everything else being the same:
I'm not sure whether this is important or not. I don't have a 2.2k resistor on hand but I have a stack of 330 Ohm resistors that came with the kit, so I strung 7 of them together to create a rough 2.2k, but circuit still did not work.
When all this was done I again connected the motor directly to 5v and it worked fine. Weird!
Here's my code:
int motorPin = 9;
int onTime = 2500;
int offTime = 1000;