The team hasn't discussed too much about the multiplayer stuff yet. Things like player limit and things that will be thrown in the multiplayer version to specificly help multiplayer version (such as General Chat, LFG, World Defense, etc.)
So everything is undecided at the moment and I cannot give too much information about this feature.
A user called "MindWorX" is programming a program called "WarSoc", it will give the power to players to click "Start Game" (single-player) and when any single-player game is started while running WarSoc, players that know your IP or IP of another WarSoc enabled map will join. Even if it's a single player file such as a campaign.
Also, this program plans to make it possible for a variety of players to play in any map of the campaign they want while others in the same campaign can play in another. When
Wc3:WoW is released, you can run from Valley of Trials to Sen'jin Village and there can be players playing in Valley of Trials, Orgrimmar, Barrens, etc. This will definately give that MMORPG feel to the game. We just hope we get as many players as possible to play this online! What've you got to lose besides some space on your hard drive? It's a free MMORPG! What more can you ask for?
And to those who wonder if we will make patches for the multiplayer version, the answer is
yes, we'll update the campaign as best as we can and remove any bugs found if possible. We'll consider these
patches rather than new versions... unless if your characters get deleted after replacing your older version of
Wc3:WoW Online.