I think it's fair to say that we're all excited at the prospect of dual speccing. The thought of switching between healing and DPS on my druid on the fly (or as close to "on the fly" as the system entails) is a great thought, indeed.

We've known for awhile, at least since our interview with Jeffrey “Tigole” Kaplan at the official Wrath of the Lich King release, that the dual spec feature will probably "be (released) in patch 3.1 that will be the first major content patch after Wrath of the Lich King is live."

Blizzard's Greg "Ghostcrawler" Street recently suggested the same time frame, but he also added another interesting bit of information: "It will be trivial to do in town. You will also be able to do it out of towns, but not as trivially."

It's impossible to tell exactly what that means at this stage, but it seems from his comment that it won't be as simple as changing stances with the click of a button out of town. You can read his full post here or below, which also touches once again on the "easy" level of WotLK. He says Ulduar will be more difficult for guilds who like a challenge.

I have discussed the dungeon difficulty issue before, so I will just bulletize it here:

Naxx is an entry-level raid.
Many players have seen the encounters before.
Players were allowed to learn the encounters on beta.

On the topic of hard modes:

I think a lot of players do enjoy the challenges, especially when some kind of badge of honor is involved.
We do realize that vanity items like titles and mounts can only motivate people so far and we do have plans to offer better loot for such accomplishments.
We aren't likely to spend several months of development time on content that less than 5% of players will ever see.
We do understand that groups like a challenge though. Ulduar will be more difficult.

On dual spec:

The idea is to let you swap glyphs and action bars along with your talents.
It will be trivial to do in town.
You will also be able to do it out of towns, but not as trivially.
We hope to have this done for the next major content patch, but it's a big feature and takes some time.