I think that the main reason in this case is development time and budget, they are a lot easy to make than a full 3d game, the game engine is more easy to handle even is the models are 3d...

Is like 2d games are now some way to teach how to make games, an step to make more complex 3d games... 

 I wouldn't say less of a game.  I'd say not graphically impressive.  The last couple generations have been pretty graphic intensive and anything that doesn't have 3-D models is commented on as looking like an "SNES" game.

So 2-D games don't get takin seriously because of most core gamers preconceived notions on what a game should look like.

I'd guess the only reason LBP is 2D is because a 3D game like it would be too ambitious for the production team. 

Well this is an indication of the future of 2D games.

A quote from Marvellous Interactive's Yoshifumi Hashimoto:

"It's harder now to make 2D-graphics games. Before, everything was 2D, so you had enough people who were actually specialized in making 2D characters. But now, everything is 3D. So now, to find a good team that can make 2D games, even if you have better technology and more RAM or whatever, it's really hard now."

I doubt it's "really hard".

anyone of us can take a class right now for a couple weeks and make some sort of 2-d game.

I’d say it’s a combination of a multitude of factors.  At first when 3D games became the mainstay on consoles many people considered it to be a natural ‘progression’ with 2D becoming associated with older games.  So 2D became more backseat on consoles to live up with those types of expectations.  Looking back that can be seen as a bit shortsighted, but it seems development just kind of got away from those types of games and didn’t really look back.  Meanwhile 2D continued on handhelds and are now looked at again as ‘retro’ games that can be re-released in downloads, compilations, etc.

Also it seems that there has been a conscious effort by console makers to encourage 3D over 2D development to ‘show off the power of the system’ due to graphic wars and the like.  I recall reading of one incident on the PS1 where Sony tried to bar the release of a 2D Castlevania game on the system for such reasons.  Also you have to consider that console games using larger screens and higher quality audio have higher ‘presentation’ expectations than handheld games.  That may lead to impressions that 2D is “worth less” due to lower production values.

I blame Sony for initially trying to bar devs from producing and releasing 2D games for the original Playstation. We also have them to thank for Working Designs now being out of business.

As for the current state of console 2D gaming, the last, best hope seems to be VanillaWare. Unfortunately, their games could hardly be called "mainstream"...

Don't forget that Mario Galaxy had some 2D sidescrolling parts

 I'm wondering too. SSBB is kind of a 2d game. The push for the "cinematic" style is fine for what it is (all that epicness) but if it pushes out 2d I'll be a sad panda.