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The edge cases happen all the time.

Using a B-Tree on a Geohash (like MongoDB does) is a bit more efficient that just indexing min/max values, but not by much. MySQL, PostgreSQL and even SQLite have R-Tree indices that perform 10x better.

If you are prepared to introduce new technology specifically to solve this problem, then you should take a look at LocalLucene, too: http://www.gissearch.com/locallucene

I've been using either MySQL with Sphinx or MongoDB with the built-in geonear successfully.

If you're already using MongoDB, it's really dead-easy to setup (see the docs).