Adafruit and Sparkfun are US companies. Since this is a GSM module it will only work in 2G GSM networks operated by AT&T in the US (please correct me if I'm wrong) who a year ago decided that they will not replacing faulty 2G GSM equipment, but let the network die slowly. I.e the module might not work in your particular neighborhood.
Since you are supposed to certify your GSM equipment with the operators in the US, and AT&T no longer accepts 2G equipment it cannot be sold in the US...
However, they are available from http://store.arduino.cc/eu/index.php?main_page=product_info&cPath=11_5&products_id=244 and I guess it should work in the US with AT&T, you might wish to try it but there are no guarantees.
3G modules are usually about three times more expensive, but maybe there is a 3G shield avavilable from third party companies? The cost of the US operator certification is still a major hurdle for the small volumes produced. (~20.000-200.000 USD) They would work, but you can not sell them on a larger scale as the US operators might decide to ban them in their networks.