The only downside is that the receiver of the business card needs a computer to see the content on it. Of course you can simply print the graphic on one side and then your usual name and address on the other side but that spoils half the fun doesn't it?!
Oh and James Aliban does a lot of cool AR stuff... like this:
More on his vimeo page
And this is too cool. I did not know you could embed the ability to phone someone while using AR, well so long as you are viewing it through your iPhone that is.