When the stack is initially powered up the Shield goes into a standby mode and requires you to press the power button on the shield to fully power up.
As I interpret the schema you can connect a digital LOW to the RESET and PWR of the SIM shield to signal it.
Hi under http://www.open-electronics.org/arduino-gsm-shield/you can find the schematics how connect the SIM900 with Arduinoand herehttp://code.google.com/p/gsm-shield-arduino/find the library
#define GSM_ON 8digitalWrite(GSM_ON, HIGH);delay(1200);digitalWrite(GSM_ON, LOW);delay(5000);
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