dougdeg, people have come up many with food safe finishes for kitchen items. A popular one for cutting boards is a combination of beeswax and mineral oil. I bought some natural varnish oil (the brand is Tried and True) to use on cutting boards, which gives a longer lasting finish.
Walnuts can be harmful to people allergic to specific protiens in the nuts, the acidic content is errosive to horse hooves if shavings are used for bedding. As far as I know the wood used for a cutting board doesn't pose any threats. I seal mine the woodwhisperer way: 50/50 General Finishes SBF and mineral spirits. It's durable and penetrates to make a good sealed up surface. Thanks for the comments. Let me know if I need to stray away from using Walnut.