This one is a shield: http://dx.com/p/manolins-sim-900-gsm-gprs-shield-expansion-b...

And so is this: http://dx.com/p/arduino-compatible-gsm-gprs-shield-wireless-...

And this (includes an Arduino Uno compatible board for $80):http://dx.com/p/elecfreaks-diy-gprs-gsm-learning-development...