I think at this point, there's probably no need for a physical attachment to let you do this. Everything is wireless now.
Also, I don't know how the 3DS' 3D would work on a TV.. and multiple screens would look sort of odd on a TV, too. It'll be cool if there's some sort of compatability between the Wii U and DS/3DS, but I think with the 3DS being what it is, there's no real need to view it on a TV rather than the dual screens.
With the DS, you run into obstacles like Castlevaina where the bottom screen handles the action and the top screen is just a map; the controller screen would have to act as the touch screen for obvious reasons but that would mean the TV gets demoted to map screen when, logically, it should be the other way around. And then there's stuff like Hotel Dusk and Ninja Gaiden where you hold the system sideways, and that ain't gonna translate to a television.
Well, the 3D obviously wouldn't work on a TV but, then, it's optional to begin with. And, with the 3DS's emphasis on the top screen being the main screen and the touch screen being extra, that'd translate just fine to the TV and controller screens. So there's no practical reason why it can't be done.
I'm for it. I bought a Super Gameboy and loved the hell outta that.
I'm all for this, but I doubt it will ever happen, since if it does any one with the Wii U will see no reason to buy a 3DS. If it does happen, the attachment will be ridiculously expensive (around $100-200), so I don't see it happening.