Why not? DID numbers are so easy (and free) to come by. The simplest method I think would be to use a free DID sip account like StanaPhone, once connected, enter in a 4 digit for which ever outbound sip you would like to use and volla... presto your connected. They do implement something like this on a Sipura 3000 but if you can cut out the middle man (being the sipura 3000 hardware) you got yourself one hell of a service...
Either way, I'm glad it's catching some attention here as this could really go from a great service to a service you can't live without (ehhmm... move over skype) If you can build this into your system, I guarantee you will change the face of telecom, voip, and mobile subscribers.
think bigger picture...