Most Center controllers don't have a highly developed sense of humor about radar limitations, but one I heard at Jacksonville Center does:
"JAX Center, Cessna One Two Three Four."
"Cessna One Two Three Four, JAX Center. Go ahead."
"Cessna One Two Three Four is 20 miles north of Jacksonville, 1500 feet, and we would like flight following."
"I would like to oblige, but at that altitude the only radar picking you up would be the highway patrol."
Barry L. Steinman