No, but there is some excellent support to roll your own
There is little official support (discussion), however with the rise of fourthcore, there is homebrew mechanical support for one. There is a recommendation to use masterplan as the basis, and some important options to consider:
Will the Dungeon be fully mapped (like the ones from the seventies) or will it remain mostly abstract like ThunderSpire Labyrinth?
Huge levels to explore or linked 5-room dungeons?
Will it embrace Old School dungeon design lore or player psychology-driven controlled encounters? Are both mutually exclusive?
Will it be a static exploration environement (keyed rooms), a dynamic dungeon environment or a mix of both?
The application of this can be found here in the "Font of Sorrows/Primal Within" (an excellent series of posts to read.)
Here's more theoretical basis on the design of a megadungeon.
How to do it
- Grab masterplan.
- Decide on the "settings" of your megadungeon. (Delve style, room style, etc.)
- If you plan on having it be delve style, masterplan is all that's necessary. You'll be connecting rooms thematically, and a simple directed graph will serve you best for the layout.
- if you plan on it being old-fashioned, use Dizzydungeon to generate the layout of each level. With 50-70 rooms per level, each level of the dungeon becomes half a tier of play. I recommend this style. You may want to make each "level" much bigger to allow for more "organic" dungeons, or keep it at the defaults if you want each room to be an encounter.
- Decide on the design of your megadungeon.
- Star topology. (Primal within mostly did this.) In this case, have your central city connect to all 6 "dungeons." With this, the rough pattern is: weak stuff (mostly) close to the city, 2-3 factions per dungeon, players are not expected to clear a dungeon in one sitting. In this, style, the leaders of each faction are sitting at the rear (usually) of each of the 6 dungeons. Every foray ablates away some of the resources of some of the factions, gradually clearing and holding rooms (represented by city-dwellers taking choke points) until the factions can be overwhelmed. In this style, you can play up faction mechanics or not, as you wish. If your players enjoy strategic play, simulate the conflicts between factions out in the dungeons and have encounters change accordingly. The beauty of the star topology is that, so long as your players are good sports and tell you roughly where they want to explore at the end of every game session, you only need to prep a night's worth of encounters like a normal game.
- Mesh. More connections between dungeons of the star model. In this, the city may not even be a traditional "city" so much as the player's faction. Could present a fascinating "organic" design. Another route to take would be to have the players be members of an undungeon.
- Linear Toplogy. Take either of the design strategies. Have an exit defined on each. And just string them along. This is probably far less strong plot-wise, but allows for a much larger feeling of accomplishment on any given night. The linear toplogy is not a war of attrition but the adventurers following the line leading to the big bad evil dude at the end.
- Daisy Chain. Sigil is an excellent example of this, and would represent an absolutely fantastic setting for a megadungeon of this nature. If you break from lore slightly and say that this megadungeon is burrowed into the outside of sigil, then players have a teeming city above them and a huge dungeon to explore underneath.
- Adapt Ptolus. Heavy city based game, said to make an excellent megadungeon. Plenty of resources out there talking about how people have adapted it. (here, here, specific mechanics here, adventure log) With the absolutely awesome ability to simply reskin enemies, it should be quite trivial prepping an encounter in Ptolus. Simply find monsters of appropriate level on DDI, reskin to what they need to look like, and run. Stick to monster vault and MM3 in general, and you'll be golden.
That should take you about an evening, mostly working on plot and backstory.
Time can be spent statting encounters as you see fit. Depending on the tastes of your players, you can tailor your prep time to what they want to play.