If you hold start + Select while booting up Super Maio Land  the game will load in its original resolution with a Game Boy border, to make it look like it's playing on an original Game Boy. If you turn up the 3D while in this mode the screen slides just a tad back, just like it was a proper Game Boy. Also while in the middle of playing if you hold L + R and then Y, the screen will go to green just like the original. I think all of the above, is freaking sweet! 

So when you're playing Super Mario Land- can you press Y to run and B to jump, like a SNES controller, or do you have to press B to run and A to jump- like it originally was?

You don't need to hold both start and select. I've done it by just holding start.

Another feature, according to an "Iwata Asks" interview is that when the battery is low the red light on the game boy border will dim.

Link's awakening does it too but changes it to a GBC. Wish it would let me pick the colour though so I can relive my old yellow GBC I had.

It's not hidden when it's in the manual. But I guess no one even notices that button...

Also, Start OR Select. Everyone insists on doing it the hard way, perhaps because of that fool from IGN who couldn't even do it successfully.

I love Nintendo's weird attention to detail.

@TooWalrus said:

So when you're playing Super Mario Land- can you press Y to run and B to jump, like a SNES controller, or do you have to press B to run and A to jump- like it originally was?

A to jump, B to run, just like a Game Boy.

No way! That is fan-fucking-tastic! Now I really want a 3DS so I can play Oracle of Ages in a digital GBC.

That's the coolest thing I've heard all day. Great job, Nintendo!