It looks like it will probably be easier/cheaper to go with the pre made ones, but it can definitely be done.
Sorry forgot to post this. This guy used the idea but created a remote detonator with the relays. He used a 8870 DTMF IC from an answering machine. The schematic and programming files and info are in the link. Have fun.
I originally contacted this company back in 2006, and since then, they've come out with some even better products than back then. They're mostly for the Industrial sector, but their products could definately be used with carpc's. I was originally looking at the SMS transceiver, but that might be discontinued. I noticed the link seems to be broken. However, the GSM Metron might be a suitable replacement, and does even more.
After typing all that, I went through my old emails and found a price list... around $500 for a unit... well, sounds like building one is what I'll probably do too.
Have fun and keep us posted!
This "Tiny Planet" project was also published in the May 2002 issue of the Circuit Cellar magazine (issue #142). Since it was published in a large DIY electronics magazine I guess there must be some "instructive instructions", but I haven't tried to make this project myself.
It also use an AVR microcontroller. By using a bigger AVR you should be able to program it to use more in- and outputs. OR maybe the descrptions of this project can make you help understand Serasidis Valilis' project better too.
Serasidis Valilis' project was based on (or inspired by) Ricci Bitti's Tiny Planet project and was first time published 5 months after the Tiny Planet project was published in Circuit Cellar.
The best website to learn about AVR is this: http://www.avrfreaks.net
There's a big forum where you can find help for anything AVR related.