For those of you needing to find simple serial communication from a Raspberry Pi to an Arduino, the following project will be of interest. Joonas Pihlajamaa has created his "PiSerial" Ardino library, which is designed for easy serial integration between the two platforms. Specifically it's a simple formal protocol to send commands and hex, byte or word-sized parameters related to that command over serial line to the Arduino from the Raspberry Pi. This would be useful when the Arduino is being used as a bridge to various types of hardware unsupported by the Raspberry Pi.
In time Joonas plans to write more code for the reverse as well. For more information and the library download, visit Joonas' website. 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: