The following comments and info comes from Shigeru Miyamoto...

- 3D polygonal visuals give players a greater sense of being part of the action
- they also allow for more freedom and choice
- some people are turned off by 3D polygonal visuals because they feel they're too complex
- "When Super Mario became Mario 64, the range of players narrowed greatly."
- Miyamoto feels the dev team made great strides in making a 3D polygonal game for everyone with Super Mario Galaxy, but this still wasn't enough for some players
- that's why Nintendo created New Super Mario Bros.
- "When asked 'what will we do on 3DS,' the answer is, of course, we'll make both (2D and 3D). They both have their own appeal."
- the 3D effect on 3DS can help make it easier to determine depth, which would greatly help those players with issues feeling out that mechanic while playing
- this could be of benefit to Zelda as well
- "gives a great feeling of existence to Mario, Link and others."
- this also goes hand-in-hand with character weight, which also is very important to making the characters feel real
- in 2D Mario..."It's difficult to tell when they will hit the player, so we were only able to use these (depth visual tricks) lightly. However, we'll be able to use this type of thing quite a bit on the 3DS."
- Iwata: "You went more in-depth than I'd been expected. This is the first I've heard of this."
- Miyamoto: "No good? Don't tell any game magazine people."

Via Gonintendo

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Malstrom done gonna shit his pants, he is

This is excellent news

Especially the 3D mario bit

and the 2D mario bit if Miyamoto promises to make it similar to Super Mario world

i hope the 3D one is not Galaxy themed, make it in Mushroom kingdom!

Awesome! Finally Nintnedo's eyes are opened.

And stop padding out the levels by making us play them five times each just to see the ending. I took it from 64, save for the last two levels when the carpet rides and waiting for the right clock times turned tedious, but it got really annoying in Sunshine. What really sets the 2D Mario games apart is you can replay the levels, but there are a lot more, and you only have to play most of them once (and that's not even taking warp zones into account).

I guess Malstrom will have to buy one then...he thought there would be no 2D Mario on the 3DS.

Since 2D Mario is so much more popular than 3D Mario I think Nintendo should make a 1D Mario for the 3DS.

Super Mario Line, it will be amazing

Why would he think that. Especially with all the money Ninty is sitting on, this is like saying the sky is blue.  This doesn't surprise me at all.

The difference between 64 and Sunshine was that you got access to 15 stages plus some bonus ones, but needed only, what, 75 stars to get to the last world? You didn't need to clear out all six stars in every stage to hit the end. In Sunshine you needed the core 49 (50 including that one you have to do at the very beginning) to proceed, so it felt more forced, because shines you got outside the core 49 didn't count towards your total (so if you had fifty, but three of them were blue coin shines, that's too bad)

He actually wrote in an arcticle that Nintendo can't ignore the sales of the 2D Mario games, and noted in an ealier article that Iwata said that more would be made.