At long last (I think a couple of people were interested), my comparison is up at
First, some nuts and bolts: These were shot with no filter on the lenses so that filters weren't a factor. Although these were shot with auto white balance on my camera, the raw files were processed so that the white balance was the same for all the images so that a direct comparison of the color handling of the lenses could be made. Likewise, I kept exposure identical for each comparison set by shooting manual exposure.
You can make your own conclusions, but the bottom line for me is that I wouldn't be able to tell the difference between pictures taken with either lens. The tamron has a bit bigger angle of view at 17mm than the nikkor, but the nikkor goes to 55mm, which makes it more useful for portraits. The tamron is probably a bit softer in the corners and a bit sharper in the center when it's wide open, but again, if I didn't have the nikkor pictures right there to compare I wouldn't be able to guess which lens took the picture.
Of course, optical quality isn't everything. The build quality of the Nikkor is impressive. I've had the tamron for two years now and about three months ago I noticed that the piece at the front of the lens (the one that the filter screws onto) is a little loose--it has about .5mm of travel. It doesn't affect the pictures, so I haven't even tried to fix it. Maybe someday... That said, the build quality of the nikkor does make it weigh almost exactly twice what the tamron weighs (764g compared to 398g).
Focusing speed is nearly the same (I haven't tested this scientifically), but I'm using the tamron that is focused using the AF screw, and I've seen people complain about autofocus speed of the newer tamrons that have the built-in AF motor. I'm sorry I can't be more help about that, but I don't know anyone who owns the newer version.
Since I can't tell a huge difference optically, I still prefer the tamron based on price, size and weight considerations.