The most straightforward way would be to put a phone in the thing that you want to control. Then just call that phone, and control away. It would take a little extra software in both cell phones, but as you say, people are doing this already.
a friend of mine did this as his final year project (mech eng dit)
he had two phones and he built a robot.
one phone went into the robot. this phone had a small board attached to it with some digital outputs on it (switches) which were hooked up to the robot.
when he rang the phone the robot automatically answered, then he could use the keypad to control the robot, (i'm not sure if its called a dtmf contoller?)
the cool thing was that the robot also had a video camera on it that would feed into the phone so that john could leave the robot anywhere, dial it up and could then see where the robot was and manouver it around the room.
it was pretty cool one day as i was working on my own project the robot suddenly came to life and started to move around the room, it came over to where i was and "looked" at me before returning to its corner. john then sent me a text message stg like "hey james i see your working away on your project, i'm still in bed but i'll be in in the afternoon"
that was pretty cool!