Regarding optoisolators, here's a recent thread on this very subject. Notice how the idea evolves as you read down through the thread. The poster is doing exactly what we discussed.
http://www.raspberrypi.org/phpBB3/viewtopic.php?f=45&t=9617
One thing I want to mention just to be sure you're aware. The voltages you'll be working with can blow an arduino to pieces if you mess up. It can also actually kill you. Yes, I know you know this and will take care, but there may be some young person that reads this thread and gets the idea to just jump in there and start hooking things together. This paragraph is mostly for them. Don't do it if you don't know what the heck you're doing, or at least have your life insurance paid up to date.