Mapping the World (opens in new tab)

It doesn't seem that they will have to contribute them back, just possibly, use the same license (which is possibly equivalent/just as bad):

> You are free to copy, distribute, transmit and adapt our data, as long as you credit OpenStreetMap and its contributors. If you alter or build upon our data, you may distribute the result only under the same licence. The full legal code explains your rights and responsibilities.

http://www.openstreetmap.org/copyright

Although IANAL, and it's not clear to me if rendering an independent set of data on top of the unchanged OSM data layer (i.e. effectively render two maps, on with some transparent sections) counts as building on top of the OSM database. So maybe they wouldn't even need to use the ODbL for their own data.

The absolute worst the OSM licence would require would be to allow people to download and distribute your dataset under ODbL. You are not required to enter it into the OSM database. At an absolute worst, you can maintain, and fully control the data, in this (ODbL licenced) dataset, without any worry of anyone changing it.

Just using NYC, the Zillow data isn't perfect. But then, the larger point is that no-one even agrees what these different neighbourhoods are anyway, so it's very difficult to come up with a definitive data set.

Has this data made its way into OSM yet?

If you draw polygons based on what you see on Google Maps (or any other copyrighted map), your polygons might be found to be a derived work of the original Google Map.

Otherwise, you could just trace all the roads on Google Maps to make your own $BLAH Maps, that's obviously not legal.