Hi,
A few days ago my colleagues and I needed to connect one arduino to a Raspberry Pi through the serial port using Rx and Tx pins. But we encountered a trouble, arduino works with 5v but Raspi works at 3.3v, so we couldn't connect them directly.
We google it and found some solutions that required to buy some circuitry. Then we decided to write a simple & cheap tutorial to make the connection.
Here is the link:
http://www.domocracy.es/connect-arduino-raspberry-pi-using-serial-port/
I hope it helps, and if you have any doubt or opinion don't be shy and comment.