He says they use Mongo for Geo queries. It'll be interesting to see if they keep using it now that the GeoCouch R-Tree index has eclipsed Mongo's geo-hash capabilities.
I work at Signal with John. I hadn't heard of GeoCouch before, so thanks for the tip. The service that uses Mongo isn't core to our application, so we're probably unlikely to change. MongoDB has been working quite well for us so far.