I have this Arduino clone with integrated GSM SIM900 module "GBoard": http://imall.iteadstudio.com/im120411004.html
I can switch the SIM900 module on and off by pressing the onboard hardware button. The "Status" LED is then on and it is also possible to send SMS or make an internet connection via different sketches. So all seems to work.
Normally you can power on or off the SIM900 module via software by a about 1500 ms pulse over the PWR pin. the documentation says ftp://imall.iteadstudio.com/IM120411004_GBoard/Documents/DS_IM120411004_GBoard.pdf it is pin D6 an the GBoard/Arduino.
So I tried
digitalWrite(6, HIGH); delay(1500); digitalWrite(6, LOW);
but nothing happened. I tried also a pulse of 3000 ms and also pin 6 to 10 because it seems there are different or wrong pin numbers on a GSM shield from the same vendor http://forum.arduino.cc/index.php?topic=80645.0 SIM900 TX/RX was connected via 2/3 and SoftSerial but I don't think that matter for this issue.
Same pulse on the Seeeduino Stalker with the GPRSBee http://www.gprsbee.com and also a SIM900 module (here you have to wire an Arduino pin to SIM900's DTR pin) is working and switch the GSM module on.
So my question: Has someone success switching the SIM900 module on by software and if so: What pin / pulse length did you use? Any suggestion how to debug are also welcome.