Worlds automatically switch to text mode if you are missing the rsc file. I don't know why anybody would be missing the world rsc file, but wouldn't it be a pitty if they were and the darn map was invisible!
Actually, I was in a circumstance like this before... someone forgot to upload the .rsc to the world, so all I was getting was the stuff I had cached a while ago (which wasn't much). I won't mention any names, though, unless they choose to =)