Chris Johnson writes:
You asked, in reference to Isaiah 40's term "The Circle of the Earth," whether this implies that the ancient Israelites believed the world was round. I'm not a "historian, archaeologist, or scientist," but I am a cartography enthusiast, and I think I can answer the question.
Mapmakers of the ancient world (see Wikipedia's entry on "History of cartography" at http://en.wikipedia.org/wiki/History_of_cartography) usually depicted the earth as a flat disk, like a dinner plate, with the ocean around the rim. Kinda like Terry Pratchett's Discworld, but without A'Tuin or the elephants. It's most likely that this flat disk is the "Circle of the Earth" the prophet is referring to.
The basic round shape of the earth (whether flat or spherical) could be inferred by observing the earth's shadow on the moon during an eclipse. The Greek Eratosthenes (second century B.C.) may have been the first with empirical evidence of the spherical shape of the Earth (see the first episode of Carl Sagan's Cosmos for a nice demonstration). Others like Pythagoras (sixth century B.C.) and Aristotle (fourth century B.C.), also believed the earth was spherical. However, this Isaiah lived well before all these highfalutin' Greeks.