|"There's something afoot in the wind"|
First, JDJarvis at Aeons & Auguries asks "How Mega is Mega?" The question being just how large a megadungeon needs to be in order to be a proper megadungeon. I personally don't think it's necessarily a question of either room count or physical size. A vast area filled with twisty passages and the occasional room can serve the same function as a much smaller, but denser, dungeon. It's about the time the PCs spend intensely engaged with the environment, rather than the size of the environment per se. If your dungeon is so crammed full of interesting things that the PCs spend an entire evening on three rooms, it's going to take them just as long to explore as if it were vastly expanded physically, and a lot more time was spent in exploration (and I don't think a megadungeon must choose one or the other; different levels or even areas on a particular level could have both characteristics). And if nothing else, check out his post for the off the wall crazy map of his dungeon.
Roger the GS at Roles, Rules, and Rolls makes a direct reply to JDJarvis in his "What's a Megadungeon?" He presents what seems to be a handy rule of thumb; each level should have twice as many x.p. in total than is required for a party of PCs to level up. With 4E's emphasis on balance and challenge levels, I'm somewhat leery of such hard-and-fast formulae. That said, I can see the value in making sure that, if you're using dungeon levels to roughly equate to character experience level, there should at least be enough x.p. for the PCs to level up before being forced down.
Next, Peter over at Dungeon Fantastic asks "How Mega is My Dungeon?" He comes up with a list of four criteria that are important to a dungeon being classified as a megadungeon: repeated play; cumulative play; diverse challenge levels; and end points, but no end. I especially like his point that "A dungeon often has an end (a demi-lich, say, or a boss fight with a dragon) but a megadungeon has accomplishments." That certainly fits with my own design philosophy regarding Castle of the Mad Archmage. Seemingly in response to A&A, he deliberately states that specific room count is not, in and of itself, a criterion. I would have to agree on that score (see above), and also with his point about factions not being necessary.
EDIT: Apparently there are even more out there than I list above. Please feel free to post more links to posts about megadungeons (keep it to within the last week, please) in the comments, and let's discuss!