I think /help answers some of it but heres what I've found out:
Since Worlds are loaded alphabetically, it's best to have some sort of identifier for them (1,2,3 / A,B,C / ...). I used A,B,C,... for my Worlds, though I keep having problems with changing Worlds(Client crash).
My World config looks like this:
# 0 = Normal Map
# 1 = Nether Map
# 2 = Floating Islands Map
# ...
# Map directory : Generator
map.storage.nbt.directories= ();
# WORLD ARE LOADED ALPHABETICALLY. you have been warned ;P
map.storage.nbt.directories += ("A-Homeworld":0); #ID 0
map.storage.nbt.directories += ("B-Nether":1); #ID 1
map.storage.nbt.directories += ("C-Heaven":2); #ID 2
This means that "/world 0" would be "A-Homeworld", but I can't prove it, because I get this:
Info from plugin.command: Command Plugin got from Xaymar: world
and then a magic disconnect with a serverside crash(Windows Server 2008 RC2 SP2).
Thats all I can tell by the current release. Sorry if it didn't help much, but I can't find out much more.