Last week I stumbled across an inexpensive electronics kit that allows SMS messages to monitor and/or control things in the home. This board has 8 control outputs and 4 monitoring inputs, and is designed to link to a cheap Nokia phone handset via a serial cable. I imagine that the phone, cable and SMS board could be bought for under $50. Add to this a prepay SIM card, and for a minimal spend, you have a system that can be programmed to respond to a range of commands sent via text message.
Given that people increasingly have electronic systems in the house that record information to computers, it would be possible to link these systems to SMS to be able to remotely control and monitor a whole variety of systems. For example, one friend of mine has an electronic weather station at home, so imagine being able to check via SMS if it is raining there. Another friend has a system that measures the power usage in the home; for him it might be interesting to be able to send a text and get a response back giving the current power usage? Perhaps from this information he could also conclude whether other members of the family are at home?
These kinds of telemetry system were just science fiction when I was a teenager interested in electronics, and even ten years ago they would have been expensive enough to limit their use to mission-critical industrial use. However, with the falling costs of prepay and SMS, coupled with the falling cost of electronics and computer systems, we can expect that these applications will become all too common in the coming years. The cellcos themselves are expecting that demand for 3G/GSM SIM cards will continue to grow in the coming years, not just for humans (who increasingly will have more than one phone), but also for robotic or automated systems that need a phone number for applications like remote control.