Will this lens fit my Nikon D3100

Nikon Fisheye-Nikkor 10.5 mm F/2.8G DX AF ED Lens…

Yes but it won't autofocus. It should be easy to focus manually though, or to just leave it on the hyperfocal distance.
http://www.kenrockwell.com/.../105fe.htm

Keep in mind though that the novelty effect of fisheye lenses tends to wear off quickly. I prefer an ultra wide angle lens over a fisheye lens any day.

I agree with Jens.

People who buy fish-eyes generally get bored with them very quickly.

Consider buying the Rokinon 8mm fish-eye since it only costs $300…

I have that lens that I use on a D90.It is a very nice lens, and while many people do get bored with fisheye lenses, good ones like the Nikon 10.5mm that have a "full-frame" image (in contrast to those cheap ones that have a round image) allow you to be more creative.

I often use to use round or curved architectural shapes that complement the lens, which provides a different perspective. And unless you look really hard, you won't automatically believe it was from a fisheye.

Lenses like the Nikon 10.5mm are relatively distortion-free along the center horizontal and vertical axis. But the fisheye effect comes from the corners. This is a lot different than the budget round fisheye lenses, which are 100% distorted.

I have used the Nikon 10.5mm to accentuate mountain tops, curves in the road, etc.simply by changing the angle of the camera to the subject. This lens opens up a lot of possibilities that the cheap round fisheyes can't.

The Nikon 10.5mm is the only AF DX lens Nikon makes. In that regard, it will only autofocus on advance DX cameras such as a D90 or D7000. But if you can put up with having to focus it yourself, it will work satisfactorily.

Here are a few photos taken with the Nikon 10.5mm. It is not apparent in the first one that the lens was a fisheye, because it follows the architecture as I explained above:

The second photo is clearly a fisheye:

Horseshoe curve in Pennsylvania - here I accentuated the mountain tops with the Nikon fisheye.

And by using "Fisheye Straighten" in Nikon's Capture NX2, you can change the fisheye effect (but you will introduce other distortion.

After straightening in Capture NX2:

So with a good fisheye, you can take more variety of photos by working with the scene and lens. If you buy those cheap Opteka or Rokinon ones, you will not be able to do these effects, and the lens will end up in the closet. The Nikon has a premium price, but it is a lot more of a lens.