What sort of GPRS modules are you looking at, the ones I've used (Arduino) for Amater Radio have an RS232 (Serial) port and therefore are (relatively) very easy to send and receive data from, the Picaxe can generate / receive the serial data and all you have to do is match the TTL output to an RS232 input/output source, for which there are many e.g. MAX232 devices on the market at very low cost, a £1 or less.
You can use two GPRS devices to send / receive data to each other, but the range will be largely line-of-sight, you canm do PAYG and hence to a mobile phone, but short range device-to-device is easy, that's as good as it gets IMO. Does anyone else have any experience of using these devices?
BTW Radio Amateurs use a similar scheme, but the resultant modulation is applied to a Transmitter usually VHF (we call it Packet) and the networks are peer-to-peer with occasional repeater nodes.
I wish you luck and it's a good idea, it will certainly work as a proof of principle.
