Simple latitude/longitude + integer database?
How about Spatiallite, geocouch, mongodb with the spatial extension, or postgis? Don't use MySQL - it doesn't have most of the spatial functions currently implemented
I would go for MongoDB. Its simple and has good support for Geospatial indexing and neighborhood queries.
http://www.mongodb.org/display/DOCS/Geospatial+Indexing#GeospatialIndexing-Querying