Something like 10% of the population gets sick watching 3D, and yes Nintendo is worried about, and that is why they have a slider to turn it off.
Whoever said they don't want to put on the glasses at theaters, great don't do it. Millions of people do it every weekend, and also use similarly filthy bathrooms in those same movie theaters. Also, exposure to various forms of bacteria is good for you, it helps build your immune system, and keep it from getting lazy. Not saying actively seek it out, but it's also not the end of the world, or even the end of your health.
@Ian: You also alleged that 3D was a gimmick to push the new DS. Yet the 3DS is plenty more powerful, so your argument is still moot. Also, the 3D is probably to further differentiate from the PSP line, which is going to incorporate touch controls and cameras, which set the DS/DSi apart before.
@BnM: Yes, but I actually meant Nintendo's whole line portable products. I actually think if only for the sake of record keeping and boasting, they should have kept the Gameboy name since that 3rd pillar thing didn't pan out, but maybe they'll still surprise us with a portable GC that also has a GB/C/A port. Edit: I said it bodes well, not that it was proof of success.
Forced 3D sucks of course, just like upscaling SD and even 720p to 1080p isn't always a smooth transition. Current 3DTVs (badly) upscale 2D content into 3D, so by the 'logic' of the doubters, obviously 3D is doomed, instead of the obvious indication that the push is proof of concept. Movies are no longer tacking on 3D (expect big things from The Hobbit and those RED Epic cameras) because they can now plan ahead, instead of retroactively because now every knows its here to stay.