I'm sorry, but this is a bad idea for several reasons.
1. MMO's require massive amounts of code and maintenance, as well as large amounts of customer support. Little kids with nothing better to do will try to hack the game, so the exploits must be patched. Currently, little assholes can only harass other players online, where they can be ignored. When players interact online in any capacity (even something so small as embarking next to a newbie and building fast to send sieges his way) you will have assholes pop up. VERY. FAST. You can't ignore another player when they actually have the power to effect your gameplay. This will require constant attention, which will require either player run intervention or Toady to spend money on full time support.
As a veteran of dozens of small MMO's and a recent escapee of WoW (it was a great game, but it's on the decline), I can tell you that immaturity+anonymity = bad things that make tentacle demons look like just "touchy-feely" guys. Some people only enjoy a game inasmuch as they can ruin other people game experience. While a single player game can support dozens if not hundreds of them before the community of the game breaksdown, MMO's can usually only support one or two per thousand players before the game just isn't fun anymore.
2. Because the game is no longer heavily moddable (unless you want to leave it as moddable when it goes online, in which case, you'll be invaded with 500 dwaves named Urist McPenis carrying AK-47's), new content must be constantly developed. Currently, when you get bored with the goblins, you can download orcs. When you get sick of burning elves, you can delete them and download High Elves. In an MMO based DF, you can't do that. Players would be bitching on the forums endlessly for Toady to "stop being lazy and develop new content". Currently, Toady is in charge of new mechanics, the players are mostly in charge of new content, and judging by some of Toady's design goals to move more into the raws he designed it with that very purpose in mind. You'd be removing one of the strengths of DF to introduce a possibly-fun-but-problematic feature.
It'd be fun, but it wouldn't work and definitely shouldn't be the direction of Toady's energy when there are more important things to do.