SIM card: $3.61/month isn't too for a Worldwide SIM - One of the best things we've been able to work out is an inexpensive SIM that can work anywhere in the world.
That's a really great question because historically you are absolutely right, it would be crazy expensive, and that's why we're so stoked about the offering.
EX: We're making a device called "Keychain Tracker" that offers worldwide tracking at the $3.61/mo rate since we only need max 15min updates. Rates are listed at http://www.sparqee.com/products/SparqSIM and we'll be launching once the Kickstarter is complete
Arduino/Raspberry PI: You can really use the CELLv1.0 with any dev board, these are just the only two we have shields for. In fact, you can actually use it with your computer using a COM port - either USB (virtual COM) or a TTL/RS232 adapter.
All this is great but a SIM card which will work in anyplace in the world? Wouldn't that be kind of expensive ?
I suppose control over the Tx/Rx would be AT commands in accord with the 3GPP standard.
Raspberry PI and Arduino as targets... these two are becoming the most popular ones and they both have their market as Arduino is more of a microcontroller right and it doesn't run an OS like Linux or Android, but Raspberry PI board does.