I'd like to know what the difference is between place_id and osm_id. I understand osm_id is some sort of reference number + more convenient for OSM to find location v. lat/lng. If this is the case, what is place_id and why, in nominatim.openstreetmap.org, do you query by that param instead? Perhaps it's internal to Nominatim? Can someone explain the differences, the only result Google spat out was in German and a bit of a mess when translated (http://forum.openstreetmap.org/viewtopic.php?id=19671).
Actually there isn't anything called an osm_id, at-least not officially.
In the OSM database, each node, relationship, way etc have a unique id.This uniqueness is only within a particular element type. i.e. there could be a node with id 1023, a relationship with id 1023 as well as a way with id 1023. These are often called the OSM ID for that particular Element. These usually will be same in all OSM databases.
In a Particular Nominatim Instance, the index will contain a unique place_id for each feature. This is guaranteed to be unique only for that instance. The same feature on different instances will usually be different, and one place_id on different instances will usually refer to different features.
Hence the place_id should be considered as a primary key only for data of a particular nominatim instance.