I'm not sure if anybody's mentioned this, but yes, Eiji Aonuma has confirmed that Twilight Princess will boast AT LEAST as many full-fledged dungeons as its legendary predecessor, Ocarina of Time.
That means that Twilight Princess will at least have eight dungeons and eight bosses (and we already know that some additional bosses are featured in the overworld, like the boar-riding Chieftan Marauder).
What's especially promising is that the tentatively titled Forest Temple, which is purportedly the first true dungeon in the game, already looks ten times larger and more complex than the Great Deku Tree. This comes as no surprise; in the same way that Majora's Mask ramped up its difficulty for those who had experienced Ocarina, so too will this game take it up a notch for those who have already met the challenge of Wind Waker (however little challenge that might have been).
Also, we've already seen that there's a ton of exposition leading up to the first dungeon, namely Toaru Village, the Twilight Castle mini-dungeon (think of it as the TP equivalent of the Forsaken Fortress in TWW), and the Chieftan Marauder chase.
All in all, it's looking good so far. IGN did confirm that Miyamoto is a lot more involved in this one, so by combining his brilliance with that of Aonuma, this should be one huge game. Perhaps it really will live up to its claim of having seventy hours of gameplay minimum...
P.S. Does anybody think that the dungeons in this game will be beast-themed, as opposed to element-themed? In the "Forest Temple," there's a huge emphasis on working together with the resident monkeys. Perhaps in a "Water Temple," you'll work with dolphins, and in a "Fire Temple," you'll contend with dragons? Link is a beastmaster in this game, after all!