I was talking about this in chat earlier today, so I figured since I went through the trouble of making them that I'd go ahead and post these pics. They show every possible way a GameBoy game can be displayed on a 3DS.
_{note: The images are the same resolution as the 3DS screen: 400x240 pixels.}

No scaling.

Scaled 200%

Scaled to fit with blur filter

Scaled to fit without blur filter

Stretched to fill the top screen

Cheers. :3

Edit: Oh, and the 3DS's bottom screen is the same height as its top screen (320x240), so they'd look the same on that, just with less of a border on the sides.

3rd picturs looks best.

super mario land will be remade with 3D right?

Nope, as far as I know only NES games are getting the 3D treatment so far. Makes sense too – they're the same height as the 3DS screen, so they can be displayed clearly at their original resolution and without any filters.

Is it not possible that the 3DS can natively render them in 400x240? I know Dolphin can natively render Wii and GCN in HD for example(that may only be possible because the assets are HD though).

EDIT: 3D Classis are actual ports anyway and display in widescreen.

now make one full screen to double (or onedotfiveble?) the fun.
edit after image was added: for some reason it looks better than i thought it would xD

It doesn't look so bad actually, but i hope nintendo will let us change the resolution like the PS classics on the PSP

@Oregano: I strongly doubt that. When you're rendering 3D polygons it's easy to make it look HD since the polygons are being drawn by the computer. In a 2D GB game, everything's pre-drawn pixel art, so you can only scale it. (see above)

Oh, and cool, I didn't know the 3D classics were ports. Those should be cool — I'm looking forward to my free copy of Excitebike.

@abgar: Done. :3

This image looks completely acceptable to me. If it's as easy as slapping a blur filter on there I think the quality Nintendo is going to release will be just fine.

The size of the pixels is uneven and it creates a distortion that is much more apparent in motion.

and it would be very apparent in some sprites and backgrounds with fine details.

_{mario land 1 is not the game with the most detailed graphics...}

Exactly. His ears and eyes and mustache and stomach and shoes would be shrinking and growing spasmodically. He'd look like a jellyfish! Same would go for the trees and everything else.

I'd be ok with a scaled game with or without the blur filter. It's still better than playing on a backlightless regular GB! Hopefully they'll let people choose the resolution they want, like with DS games or GB games on a GBA.

this. hooray for no more eye strain and/or fiddling with my old Cobra attachment for light, lol :3

No scaling is the option for me. I generally don't like the look of resized 2d graphics, filtered or not.

Edit: What'd be even better, though, would be the option to choose and set a default preference! And for the DS games. Stupid that you have to hold down those buttons every time.

I thought that the 3DS screen was actually high enough resolution so that it could fit GB games scaled 200%! Oh well (I knew nintendo should have made the 3DS have 1.5x the DS' resolution...)
But in this case I would be great with the original resolution or scaled to fit vertically w/ blur filter, they both look fine to me, and the blur filter makes the graphics look a lot less blocky and pixelated.
P.S. What was the original GB's resolution anyway?

160x144. :3

Why can't super mario land be played in color? Like with a game boy color?

@soccer686: i don't really think they've said yet whether we'll be able to choose a color palette (like with a Super Game Boy or GBC) or not, have they? i'd think they'd put that functionality in there (but then it wouldn't be the first time i'd ever thought wrong, haha)...

Third image looks the best to me, but at least you have several options of screen settings for downloadable games (unlike the DS games).