Nel wiki ho letto questo:
When using GPRS Shield with Seeeduino Stalker v2.0 please remember to dismount the OK_READ Jumper (i.e. open it). This will disconnect the Battery Charger IC's OK pin from the microcontrollers Digital Pin 7 and hence allow unhindered communication with GPRS Shield using NewSoftSerial Library.
Poi dovresti controllare i livelli dei segnali, perché Arduino lavora a 5V, la Stalker a 3.3V e il GPRS a 2.8V. 