1. A method, comprising: performing, by a computer system that stores a plurality of objects in a data store, each of which comprises a user key that represents an identifier of a versioned object and a version identifier that identifies the object as a particular version of the versioned object: receiving a request to retrieve a data object from the data store, wherein the request specifies a given user key, but does not specify a version identifier; and in response to receiving the request to retrieve a data object: determining a version identifier for the request, wherein said determining comprises determining the version identifier of the latest object version stored in the data store that comprises the given user key; and initiating a retrieve operation, wherein the retrieve operation specifies the given user key and the determined version identifier; and returning results of the retrieve operation.