Just a quick mind share: Here is a new GSM connectivity alternative compatible with the GSM Shield. It’s called Neo, an online marketplace for IoT. You can get GSM sims, manage devices, etc., with an affordable rate plans (no binding contracts).
Community Forum for Neo: http://goo.gl/0ZispB
Main Neo site: http://goo.gl/IqScpX
Note: this is ideal for those who are testing products they wish to bring to market.