Jon Shafer, a.k.a. "Trip," is a veteran Civilization modder and mapmaker -- he's made his name in the Civ community by posting his own mods and strategy guides on the forums for years. And oh yeah, he's also the lead designer of Civ V, which is probably one reason why user-created mods and maps will play such a big part in the official game. Players will be able to upload and share them with others directly in the game itself.
The user-created catalog will even be populated with plenty of content right at launch, because Shafer tells Bitmob that "you can import Civ 4 maps into the world builder and convert them into Civ 5 maps, including all the units and cities and stuff on it -- the conversion process will just do that for you automatically." Presumably, the process will also translate the maps from squares to hexes, which means Shafer and everyone else's maps from the last Civ are just a conversion away from being playable in the new game. Which is good -- some Civ fans would have a tough time without their Middle Earth maps.