So it started with Dungeon A Day by Monte Cook. He's writing up a megadungeon for D&D 3.5, with one bit of encounter a day. Nifty, but wrong system and price tag (7$ a month) for me. Not that I doubt Cook will get subscribers, because he's Monte effing Cook.
Then the blogosphere exploded with counter-offers. James Maliszewski is starting up a collaborative megadungeon project (with himself as benevolent dictator), probably for OD&D. Chatty DM is musing about a similar project for 4E. I'll be watching both.
Anyway, I had a similar idea, inspired by West Marches. Someday, I'll cook up an "adventure setting" for D&D 4's Heroic Tier (level 1-10). It'll have regions (similar to the levels of a dungeon) balanced for a three-level spread (so the relatively safe lands around Hommlet might be for levels 1-3, with threats in the 1-5 level range). In each region, there would be some short dungeons like the three-encounter delves in Dungeon Delve and some longer ones depending on what makes sense for each.
Oh, and I'd add a lot more than needed to level up a group to 10:th level. That makes PC:s prioritize and gives the players freedom to choose what they really want to do.
All this is subject to change, of course. I'm easily distracted.