I want to exchange some data between two android phones. I realize I can do this over WiFi and Bluetooth but their range is much limited as compared to GSM. I'm wondering if I could do this directly using the GSM module.
As far as I know a SIM's function is not to provide transmission but only identification. It allows phone to authenticate with a provider and decrypt the signals sent on that network. The hardware to transmit and receive signals is there in the phone. So if a phone is not registered on the network it would still receive the signals going around it (though for the sake of efficiency it may choose not to do so if unregistered) but not understand it since it can't decrypt it.
Now in light of above, two phones that are in range of one another could directly transmit and receive with each other if programmed to do so. And the range of transmission would be quite good.
Is it possible to program an android phone to transmit data over GSM without a SIM?
Note: I realize what is being asked here may not be implementable straightaway using standard Android SDK. But this is for experimentation and DIY projects so I'm okay working with rooted phone and performing hacks.
No. It is not possible. The Radio Access Network technology, GSM or CDMA is only one part of cellular technologies. There are two main parts in cellular technologies: the Access Network (BTS or, NodeB (UMTS), RNC (UMTS), and RAN technology) and the Core Network, where, in GPRS and UMTS, there are components like Service GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN), among other functional components.
In order to send data, first, a cell phone needs to establish a Packet Data Protocol (PDP) context, which involves data stored in the SGSN and the GGSN. For this the IMSI, stored in the SIM, and TMSI are necessary. If the device cannot establish a PDP context, it won't be able to send data.
You can read more in about UMTS Network Architecture Specifications in http://www.3gpp.org/ftp/Specs/html-info/23002.htm
In LTE, it's the same case.