Yep, I asked AT&T why my monthly payment didn't go down when my iPhone 4 was paid off and they wouldn't give me an answer.
Depending on how much the monthly plans are, this might a good way to attract customers. Your monthly plans SHOULD be lower if you use an unsubsidized phone. Isn't the point of a contract is that you pay for the phone over the 2-year contract? And when the phone is paid off, your monthly should go down.
Not at AT&T or Verizon.