I think it was a caller to Rush Limbaugh who confirmed the presence of this billboard on I-35 between Minneapolis/St. Paul and Duluth MN. (In fact, she sounded like a relative of mine, except said relative would be a little upset--on the political spectrum she's quite left.) Through the wonders of the interwebs, we find this photo and this article about it. You should read the comments with the article. I am not surprised that there are many liberal comments condemning the billboard--Minnesota is basically a liberal state. However, conservatives come to play too, and make several comments.
In fact, if the billboard and article comments show anything, they show how polarized this country is politically and how hard it is to talk to each other about issues.