select for update

I thought I understood "select ... for update," but maybe not.

We have a number of separate databases and a unique integer identifier that's supposed to be global across all
databases. A single "archive" database is used to issue the next available ID when a process wants to create a new
object. The sequence of operations goes like this (pseudo-code): 

   /* start a transaction *
   begin;

   /* see if an objectid has been returned for re-use */
   select objectid from archive where db_id is null limit 1 for update

   /* no ID available?  issue a new one */
   if (objectid is null)
     new_id = select nextval('object_id_sequence')
     insert into archive(objectid, db_id) values('new_id', 'new_id')

   /* ID available? claim it */
   else
     update archive set db_id = this_db_id where objectid

   commit

The problem is that very occasionally the same ID will be issued twice.  I don't see how this can be.  Doesn't the "for
update"guarantee that no other process can claim that same row? 

Thanks,
Craig