Seeking neighborhood boundaries globally?

I don't know what the state of mapping with this is, but OpenStreetMap has a place:neighbourhood tag (note the British spelling).

As the boundaries of neighborhoods are often nebulously defined I think no matter the source you will have to understand that using them for anything more than general display/labeling is going to be tricky.

Check out commercial data provider http://www.maponics.com for neighbourhood boundary data.

How about the Flickr Shapefiles which are constructed from the location tags people give geolocated pictures on the site.