* Implementation: Access to the DNC list is provided by various carriers for paying customers, with lookups available on a per-number basis. A binary { true, false } response is sent. EDIT: Thank you PaybackTony for the correction. The entire DNC list is also available as a CSV for $$$$.
* Expectations of privacy: Does Maxmind send out a response to those fulfilled complaints stating that their IP address will be made available to anyone on the Internet who registers for an account?
* Hiding in the crowd: How many numbers are in the DNC registry? How many will request a Maxmind opt-out?
Regarding trying to identify them from it would in itself be illegal probably?: maybe. But with all these IP addresses sitting available to anyone on the Internet who bothers to make an account, it doesn't really matter.