I too am interested in this (funny how I encountered this problem exactly 1 year after the OP).
What I don't get is why exactly is this so hard to port to the Due? From what I gather, it's just a software serial connection and a reset pin. Assuming all the GSM library does is send a series of commands to configure and operate the shield, couldn't we just use establish the RX1/TX1 serial connection and send commands manually? How complicated could it really be to tell it to send a text message, pick up an incoming phone call, or get the PIN? I don't have any documentation on it so I don't really know where to begin. Skimming through the source code to the GSM library, I can understand why nobody has bothered to support the Due - it's pretty big. On the other hand, if this really is as simple as just sending serial messages with some sort of header, that library seems very over-complicated.
While this is a different board, wouldn't something like this be possible:http://www.seeedstudio.com/wiki/GPRS_Shield_V2.0#A_Simple_Source_Code_Examples_for_phone_calls_and_sending_text_messages