Hello,
I'm looking to buy a SIM900 based shield - http://www.geeetech.com/wiki/index.php/Arduino_GPRS_Shield - but I'm not convinced by some lack of information...the wiki page just says to plug the shield and it would work "out of the box", but checking the schematics I don't see any logic level shifter - If I'm not mistaken the SIM900 works at 2.8V - max 3.1V... (http://itead-europe.com/pm/platform/shield/icomsat/DOC_SIM900_Hardware%20Design_V2.00.pdf pag 37)...in other words an Arudino at 5V is sending serial data to a SIM900 rated 2.8V....
anyone had any experience with this board?
I can't vouch for that particular board but these shields use a power regulator to convert the input voltage to the SIM900's required voltage.
basically, the shield will "just work" 
OK for the Vin - it is regulated...and it is taken in fact to 2.8v..
but UART seems to remain also at 2.8...