Spawn a map item. Then go into the server's world folder->data, and delete the new maps' files. (like if you want to clone map_3 as map_0, delete map_3) then, copy and paste map_0 (or whatever map you're cloning, and rename it to the new map (example, map_3) Notch may add a cloning feature in the future, though!
The first map created is map_0, the second created is map_1, the third created is map_2, and so on, up to map_65535. Note that each time you zoom out "one" map, it actually creates a new map with a new map name.
Aside, you can give your maps more memorable names by renaming them in the anvil.
Best Answer
Now you can change the maps using an anvil. It requires levels to rename the map however.