Fascinating topic. Would be interested to learn how it went with Bubbz's test.
I wonder how many dots of color are on a billboard, or on the side of a truck used as one. And whatever does that mean when translated into a requirement for pixels at the sensor plane? I realize that folks will see the side of a truck from a lot closer than they will see a billboard alongside the highway. I am guessing that the D700 would be a safer bet for the billboard than for the truck.