I have one of my SE phone always connected to my computer, which is receiving text messages. I just thought about why wouldn't this application receive commands along with messages. So I can send SMS "Shut down" to my phone and computer would shut down.
You could add 3 text boxes, where user can enter from which phone command is accepted (or all), command and action (Shut down, Restart, Run file - this is most important, with this you can then run batch files or other applications - this is main function, others ca only be xxx extra, since all commands can be executed if you have run command available).
Here is example:
In future this could develop further in a way of more functions, like call back (I send sms "callme 10" and after 10 minutes phone calls me - good if you need to get a reason to leave when you are bored), voice calls (I send sms "call 555123456" and some voice in MP3 is played to the one who is called - or maybe "call 555123456 one" and one.mp3 is played if it's possible to play MP3 files over phone. I know I had that on my W800 a few years ago),...
I think that this would be one of the best functions and would open a whole new usefulness (and it won't take to long for you to develop - the most time you will spend on form).