Just found a way to create my own Neighborhood database from 2 data sources. One is a complete file of US Zip codes with Lat/Long Centroids. The other is a free file from the USGS found at http://geonames.usgs.gov/domestic/download_data.htm
The good news is that the USGS file has a Feature Type called "Populated Places", which in urban areas contain neighborhood names. I tested against Urbanmapping and for San Francisco and Atlanta it had all but 1 of their neighborhood names (via Neighborhood API demo).
Since both the zip code file and the USGS file have lat/longs, the only thing left to do is to associate the neighborhood names with the zip codes. To do this you can use the following equation and compute the distance between all zip code lat/long and all Populated Place lat/long, then do a select based on the distance you want to include. I am using a 5 mile radius for the zip centroid. Here's the equation:
((ACOS(SIN(‘.$latitude.’ * PI() / 180) * SIN(latitude * PI() / 180) + COS(‘.$latitude.’ * PI() / 180) * COS(latitude * PI() / 180) * COS((‘.$longitude.’ – longitude) * PI() / 180))* 180 / PI()) * 60 * 1.1515) AS distance
Hope this helps other people struggling with the same issue and having a hard time justifying the cost of licensing this data (which is huge for a start up).