Step 1: Shut Off All Serial Action Going on With the Pi.
Then bounce over to /etc/inittab and we have to comment out the last line by adding the # to the beginning.
#T0:23:respawn:/sbin/getty -L ttyAMA0 115200 vt100
Reboot with your Arduino plugged in and make sure you have the proper power.
Step 2: Download a Serial Terminal Like Minicom or Cu. I Like Cu.
CU is a serial terminal viewer that allows you to directly send keystrokes over your serial port.
Step 3: Figure Out Where Your Arduino Can Be Accessed.
lrwxrwxrwx 1 root root 13 Dec 31 1969 platform-bcm2708_usb-usb-0:1.2:1.0 -> ../../ttyACM0
Step 4: Enjoy the Magic of Sending Bits Over the Wire.
cu -l /dev/ttyACM0 -s 9600
and you should see a connected message
Visit http://thomasloughlin.com/serial-connection-between-raspberry-pi-and-arduino/ for sample code and more in-depth details on the connection.