The OnRebate CEO, Bill Gollan, did respond to an email. He promised to straighten things out, and it appears he has done so.
Bill was also nice enough to give me his direct-line phone number. I spoke to him for a little bit and he seemed completely interested in helping me and listening to my concerns.
Hopefully this straightens everything out and I will get my nice little check for $5.50.
UPDATE (4/11/2007): Yesterday’s mail brought my refund from OnRebate. Nice guys that they are, they even rounded up and gave me an extra fifty cents!

Bill spoke with me after your call today. I had escalated the issue to him this morning. Glad to hear all is well.