If you're looking for a way to remotely control an Arduino and you don't have any Ethernet hardware, this solution by Marco Schwartz may be of use. He's demonstrated how to control an Arduino remotely by creating a mix of python, PHP and Arduino sketch that uses a host PC as the Internet connection. His particular application shows how to control a DC motor, but with a little work you can modify it for other tasks such as digital output control and so on. Furthermore Marco's article is quite detailed and actually explains the sketch in detail, so it's a good read.
To find out more check out the Open Home Automation website. If you're interested in home automation, also check out SuperHouse.tv. And for more, we're on twitter and Google+, so follow us for news and product updates as well.
If you're new to Arduino, the first step is a solid board for your projects - our Freetronics Eleven - the Arduino-Uno compatible with low-profile USB socket, onboard prototyping space and easy to view LEDs: