Kinda took this from the metaworld topic into a seperate topic...
A real-time multiplayer game (dedicated server-wise) would be possible, assuming that the world would always run at a set speed. And it'd be just as possible to make dwarf/adventure mode in one world interact and all - all as long as the game speed remains constant and cannot be paused (meaning that building and placing stuff and performing all sorts of tasks would be done in real time). The only thing is that the gamespeed would perhaps have to be brought down a notch for comfort but apart from that there's nothing else that I can think of that would need changing.
And besides, that would be so frigging awesome - imagine fortress-players trading, forming alliances or engaging in wars and all that stuff and in the middle of it all you have adventure players who roam about the map and who can get themselves involved if they so wish.
Of course, the game WOULD have to be based on "1 life and that's it rule-set" - specially for adventure mode players. Your character(s) die, then they die. No respawn or anything.
That way you could have rankings and so on - see how long an adventurer can survive or how great you can make a fortress etc.
I mean, one world is huge enough to support quite a few players, and I'm sure that it'd be possible (if needed) to have even bigger worlds.
Heck, this could even become a completely new kind of MMOG if one could have a decent enough server to run a world, cause as far as I know, this kind of gameplay idea is rather unique (as of yet).
Of course, before one can really start going towards this direction I'd prefer to see the actual game in a more finished state.