That's the tutorial I wanted to follow! :)
If I understand what is explained in the tutorial, there's no voltage issue if the Pi is the master and the Arduino the slave.
But as I tend to bump on each "that's not going to happen" in my experiments, so I'll be carefull and find the way to protect the physical connection ;)
In my case the I²C connection worked fine (except of the problem with the onRequest on Arduino not getting triggered via i2cget) when connecting the SCI/SCL/GND directly to Arduino/Raspberry Pi GPIO. :)
I really have to post my trials because I think I could use some experience from your build.
I'm building from a DFRobot 4WD with a Romeo board (arduino) and I'm planning to add on top a Raspberry Pi to have some OpenCV stuff.
Could you explain what you used to interact between your 2 boards ?
This was a little bit tricky, since i wanted to use PHP as the server language on the Raspberry Pi.
My first try was to connect them via I²C - connected via the GPIO pins (RasPi as master, Arduino as slave), here is a quite nice tutorial: http://blog.oscarliang.net/raspberry-pi-arduino-connected-i2c/
I've written commands via a bash-call "i2cset".
Since i could not read over I²C for some reason (i2cget just didn't triggered the "read"-event on Arduino) i started to connect them over a USB cable. (Serial communication)
Here is a quite nice tutorial as well: http://blog.oscarliang.net/connect-raspberry-pi-and-arduino-usb-cable/
The only thing i had to add was a bash call after every boot:
# stty -F /dev/ttyACM0 cs8 9600 ignbrk -brkint -imaxbel -opost -onlcr -isig -icanon -iexten -echo -echoe -echok -echoctl -echoke noflsh -ixon -crtscts
After this call the serial communication worked quite fine, i'm using simple fopen/fread/fwrite commands from PHP to access the /dev/ACM0 usb device.
P.S.: Do not power your Arduino from an external power source when connecting it via USB cable with your Raspberry Pi because it will get powered over the normal USB port without a fuse.
If you are having any problems you are welcome to contact me!