Single player games like Fallout3 or The Elder Scrolls serie wouldn't work well as MMORPGs, as the mood and feel of the original games would disappear completely.
When you play them alone, everybody else (the NPCs) are actually roleplaying, and you do it more or less because it fits and because nobody is watching. You can go to Moira and lie to her about finishing the mission she gave you, you can go to your house in Megaton (provided you didn't destroy it) along with your faithfull Dogmeat, ask for a joke and a drink, rearrange your home, then exit the city and be immersed in the desolate landscape, perhaps glimpse raiders running fighting off a mole rat in the distance, but that's it, while 3dog tells everybody about the guy from 101 who did that or did this.
Enter MMORPG players. The NPCs are still roleplaying of course, but who cares? The chat channel is full of OOC talk about some football team winning a game, which new MMORPG is coming out and why it is better or svcks more than FO, while someone explains to a new player that, to get the shishkebap, you need to go to "loc 1400/400" (click on the line to set your pipboy automatically, talk to the NPC called Mr.Smith, click the first answer and then the third answer (no need to read it anyway). All that while people fill the trade channel with "selling 100 motorcycle brakes for only 200 caps".
You get out of Megaton, and you see groups of players (two tanks, one long range guy and a doc ... Basically they could be two dwarf warriors, one elf ranger and a high elf mage for all it's worth) standing at the entrance to some Vault or running to deserted houses containing instanced dungeons. If it's a PvP, you'll probably be spammed by duel challenges from lvl1 guys wielding fatmans and shishkebaps and carrying 150 stimpacks. In short, the atmosphere is gone. It's like any other MMORPG out there, just using FO graphics, and what made the world of FO3 so entertaining as a single player game is just not there anymore. IF nobody is roleplaying, then it doesn't matter what the game looks like or where it takes place anyway. You can ignore the chat channels of course, or just listen to some dedicated "RP" channel, but then : why do you need to play it online anyway if, in the end, you're still playing it as if it was a single player game?
I'm not completely turned off by the idea of an online version of FO3, but from my experience, it would be best fitted for small scale multiplayer running on private servers (not quite unlike the online version of System Shock 2 for example, or Neverwinter Night), but I think anybody who thinks a MMORPG version of Fallout would keep the fun of the original game is quite mistaken.
random rambling, I know ...