There was at least one or two utilities that used DF Hack to run a sort of multiplayer thing.
Dwarf term, and all of it's compatriots caused innumerable crashes. It really wasn't worth it.
The game expects only one person to play and attempting to have two or more people playing the game at one time, destabilizes the internal workings of the game.
It isn't a matter of 'oh it's only more work, and toady one does work'
It's a matter of scrapping the entire game as-is and building it from the ground up.
Hiring (more) people just to test,
Actually testing the game with multiple people at the same time, with enough frequency to catch most of the bugs,
Chasing bugs that only happen when multiple people are playing.
Making features now designed to work for multiple people using Dwarf Fortress at the same time.
As well as complicating the addition of every other feature.
Not to mention all the features and secrets that Toady One would have to modify or outright remove (There's gold in those hills!!!).
With the heralding of the steam version, such a thing would force the new, paid, audience of Dwarf Fortress to sit far longer than Toady One is comfortable with.
It is an utter impossibility given the constraints.
The bottom line is that online/lan multiplayer will never be added or supported.
You can however, share saves with other players for play by post/play by email.
Saves were cross compatible in previous versions, so once the mac and linux releases are out, those should also be cross-compatible with pc.
Sometimes, a person starts a fortress. He plays a little bit and post it for the next person to work on. Then that person builds some and passes it on to the next person. So on and so forth.
As well, sometimes a single person spends months of their own time building a fortress, and then when it falls, or they tire, pass it on to another person to explore in adventure mode.
We have a whole section of the forum specifically for this:
http://www.bay12forums.com/smf/index.php?board=14.0