This is really cool! But also kind of complex for a task that could be performed in a much simpler way.
Using a GSM module such as the Telit GM862 [1], you will be able to receive text messages directly on the Arduino without the need of any web services. This would be much faster and cheaper. This particular module even includes a GPS.
Indeed - I've been meaning to experiment with a GSM module actually, I just had the WiFly handy so I built it with that instead. One thing I would say is that using the WiFly allows you to connect with web services as well as the cell networks (in my example I made a very basic Sinatra app with an on/off button)
It's definitely also a good argument to just use what's in hand!
It is actually also possible to connect to the internet using that module, using good old GPRS (The module contains a full TCP/IP) stack. Of course not near as fast as an average wifi connection.
I used it to create a set-up where you can send a text message to the module: "Where are you?", it would then respond with the latitude and longitude as well as the geocoded address using Google Map's Geocoding API over GPRS.
The Telit module is fully on the internet just like your PC. The module can do GET's and POST's directly to any server - no extra host needed anywhere.
My understanding is that you get raw SMS data similarly to raw TCP data - via Serial assuming your GSM shield supports that sort of functionality. You would likely need a prepaid sim card as well, but it is possible to get SMS directly to your device without any intermediary server.
Personally, my comfort zone was with web development and I had the WiFly handy so I used Twilio and Pusher - worked out alright :)
I think that was notatoad's point. To use GPRS modules, I would think you need to buy access to the towers from somebody. I don't see how that would be cheaper than using WiFi, because you would have to pay for every communication.
Using a GSM module such as the Telit GM862 [1], you will be able to receive text messages directly on the Arduino without the need of any web services. This would be much faster and cheaper. This particular module even includes a GPS.
[1] http://tinkerlog.com/2009/05/15/interfacing-arduino-with-a-t...