They're not hashes. There's a few things embedded in the ObjectId (from the docs - http://docs.mongodb.org/manual/reference/object-id/):

  ObjectId is a 12-byte BSON type, constructed using:
  
  a 4-byte value representing the seconds since the Unix epoch,
  a 3-byte machine identifier,
  a 2-byte process id, and
  a 3-byte counter, starting with a random value.

You can actually convert the _id to/from a timestamp, which lets you do cool things like never keep a timestamp field (you can convert a datetime to an ObjectId and use that for comparison).