Argument: If Satan showed Jesus all the kingdoms of the Earth during his temptation (Matthew 4:8 and Luke 4:5) then the Earth must be flat in order to be able to do this.
Reply: if this is to be taken to be Satan taking Jesus somewhere physically then it would mean that there is, according to Matthew and Luke, a mountain somewhere from which you can see all the kingdoms of the world. The result of this is that wherever you are in the world you would also be able to see that mountain. Are we really to believe that people never noticed there was no such mountain? When on a boat in the Mediterranean, are we really saying that sailors never noticed there was not at least one mountain in view?
Sounds to me like Matthew and Luke are telling of a vision or something like that.
Update: an atheist has just told me on the BBC messageboards that sailors stuck close to shore in those days and thus would not have noticed this. I have pointed out that people must have sailed to Cyprus to get there and, if my calculations are correct, in order to keep the mainlaind in sight all the way there, they would have needed a mast 1450 foot high with some poor unfortunate on it to keep an eye on the land in the distance.