Never heard of a card but I made up a "Yes Open carry is legal" trifold that I keep several of with me to hand to people. Some are interested in learning more, if I come across an anti I'll hand them one as well.
It's printed on both sides, has some of the pertinent laws (almost one whole side), frequently asked questions (with answers), some links to info on the net, and an ad for the second amendment march.
Not as durable as a card but has a ton of info.
I also carry two different "cards". One has my first name, e-mail, and says "open and concealled carry advocate". The other says "no gun = no money" and a paragraph about the growing open carry movement and that their business will be posted on several internet sites as ANTI GUN so that readers know not to shop there.