zillow distributes free kml files for the neighborhood boundaries.

Edit: here is the link http://www.zillow.com/howto/api/neighborhood-boundaries.htm

Yeah, I've looked at those a while ago and it seemed that few cities, particularly NYC, the data was some what low quality. It was either really lacking in certain areas or the neighborhood boundaries were far off from what people who actually live in the city consider to be the boundaries.

Although perhaps they've gotten better, I'll give them a look.

That link is old, but another resource to look into is http://www.openstreetmap.org/

Is there a good implementation guide for using the kml data to lookup a neighborhood given a lat,lng? I considered the Zillow files prior to using SimpleGeo, but implementing a spatial database in MySQL seemed very complex in comparison to a simple CURL request. Perhaps, this is worth another look.