I'm on a unlimited family plan, and it's not working here either.
If you're on at&t with a Messaging 200, Messaging 1500, Messaging Unlimited, or Family Unlimited with the correct ipcc (verify this with Xcode's Console giving the 403 Forbidden error instead of some other mms_proxy or other error), can you post if you requested your device to be "provisioned" (their words) for MMS?