For those of you enjoying your Raspberry Pi so much that you'd like to use it as the host PC for your Arduino projects - a simple project by Michael on the mitchtech blog is the answer. He runs through the procedure to download and install the Arduino IDE, and also notes that you need to power the Arduino board externally as the power circuitry in the Raspberry Pi cannot support itself and an Arduino board simultaneously.
If you're looking to get more out of your Arduino projects, but find the normal Uno-style boards limiting - check out the Freetronics EtherMega:
Quite simple the EtherMega is the fully-loaded Arduino-compatible board on the market today. Apart from being completely Arduino Mega2560-compatible, it includes full Ethernet interface, a microSD card socket, full USB interface, optional Power-over-Ethernet support and still has a circuit prototyping area with extra I2C interface pins. So if your project is breaking the limits, upgrade to the EtherMega today.