Here’s a great example of someone who used Twitter to control his home appliances. Of course, this is completely do-able with some meticulous planning. I do see more and more automation DIYs using Twitter as communication platform for their projects, very useful indeed for our cluttered world. You will have to add some security measures to protect against hackers though.
The one thing lacking in most of these devices was 2-way communication between Twitter and the device. In all of the examples I could find the device is sending an update to Twitter but is never performing an action based on a Twitter message.
With http://twitter.com/MattsOffice you can send a command by placing a keyword in a @reply message. Valid keywords are “light”, “bright”, “dark”, “temp”, “hot”, “cold”, “lcd”, and “view”. For example, to retrieve the temperature of my office I just need to update my Twitter status with a simple question like “@MattsOffice What is the temperature?”.