This isn't sending SMS over the radio network, it's hitting TelAPI over (presumably) the land line. A jammer would prevent the reception of the SMS at the site, which wouldn't harm this application because the whole point is that the owner is not present and needs an alarm.

It is, however, vulnerable to the outrageous latency problems (minutes, often) the unofficial messaging gateways have. I send my wife texts to her company blackberry because it's convenient, but ultimately had to duplicate them to her gmail address to ensure that she got them fast enough.

You're right there about the jammer. But in testing, it hasn't taken more than 20 seconds so far.

This wouldn't be vulnerable to that, as the SMS is sent via a web-based API. You'd have to block the Arduino's wifi/ethernet connection.

I don't think the meth addicts burglarizing my neighborhood are that sophisticated. ;-)