This billboard appeared south of Mustion Creek on the west side of U.S. 63, in West Plains, Missouri according to the West Plains Quill.
The Springfield News-Leader picked up the story and now it's getting wider coverage. It was picked up by Salon.com and someone posted it on I-Report at CNN.com, although I can't find the link now.
A news report in video can be viewed at OzarksFirst.com. The news story is preceded by a short advertisement. Neighbors report that they saw Ronnie Ford putting the sign there. The West Plains Quill talked to Mr. Ford earlier and he claims he doesn't know who put the sign there even though it's on his property.
I grew up in the Ozarks (Shannon County, Missouri) which was an extremely low-income area. I attended school with people who lived in one-room shacks with earthen floors and I recall that when an African-American man hitchhiked into Shannon County, the sheriff picked him up and drove him to the county line "for his own protection".
I have relatives there who are voting for Barack Obama and they wish they could show their support by putting an Obama-Biden sign in their yard, but they're afraid someone would retaliate against them, someone like the person who posted this blatantly racist billboard. People there are openly saying they will not vote for a black person.
OzarksFirst.com reports that Ronnie Ford owns the land where this sign is posted and neighbors saw him place it there. [Deleted address.]
Update: The Freepersare commenting about the billboard.