Tobold brought this up today, and just yesterday my gaming buddy and I were talking about it as well. We see the concept all over the forums, even hear it from the mouths of developers. But we haven't much proof of the concept except in the Ryzom Ring for "The Saga of Ryzom", a little played MMO that next to no one knows or cares about.
In our offline games, admittedly less prone to player-anarchy than in their online brethren, we see several games thriving for many years past their release due to player-created content. Neverwinter Nights and The Elder Scrolls come to mind. These games have such robust toolsets that are easy to learn and hard to master, that there are online communities devoted just to the creation of new content. These are games that have the magic 40 hour playtime, but their lifespans last much longer than one play-through due to the dedication of their fanbases.
Imagine then, an MMO that gave players with the imagination and devotion the chance to create their own missions. Create their own dungeons. Create their own bosses and items.
Of course if an MMO were to use this there need to be a good chunk of people in line to filter content and make sure nothing objectionable or "cheatastic" got through. But if the toolset was built accordingly, this could be taken care of from the word go. If you give your players enough freedom to create this kind of content, you'd be surprised how far they can take it. Evidence of this is all over NWN, TES, and Titan Quest. Even more so with the Unreal series of FPS games. Mods are what make PC games more worth of that 50 dollar pricetag than their console counterparts.
Yet, as MMO gamers we pay the 50 dollar price, we pay 15 dollars a month, and we're happy with our every so often updates and paid expansions. But imagine the retention rate of the game that right from the launch has a robust toolset to craft dungeons with and share the creation with the masses. Imagine being part of that. Imagine a world that really does always have something new to try. No "Man, I can't wait for the patch to come" because if you want new content you need only make it yourself, or go searching for someone else's newest addition.
There are plenty of limitations I know that would need to be put in. Not to mention the time it would take to make a user-friendly world builder that anyone can use. But I truly believe the end result would be worth far more than the effort it takes to get there. The MMORPG as we know it could be altered by doing what every developer says they want to do... put the world in the players' hands.