using a cmos cd4050 buffer chip to convert 3.3v to 5v.
the raspberry pi has its UART run at 3.3v and the arduino needs 5v. so, this little DIY perfboard 'shield' is needed.
note, how the chip is specifically (yes, very much on purpose) resting on the unused white riser connector. there is no other support (or screw mounts!) on this version-1 raspberry pi board.
schematic link1: picasaweb.google.com/lh/photo/j-l64p2I_4QVANbyiK8JFdcYewr...
schematic link2: www.andremiller.net/content/raspberry-pi-and-arduino-via-...
discussion link (AMB diy forum thread): www.amb.org/forum/raspberry-pi-lcduino-system-t1970.html
this board was so simple, I actually didn't run out of wire-wrap wire colors ;)