The documentation on https://api.drupal.org/api/drupal/includes%21database%21schema.inc/group... states
'serialize': A boolean indicating whether the field will be stored as a serialized string.
But this doesn't work with any of the new db_* functions. (i.e. db_select, db_insert, db_update or db_merge). It does however work with drupal_write_record() for serializing.
Based on what Crell wrote in this comment https://drupal.org/node/585894#comment-2781998 it would seem that the intended functionality is to NOT do the serialization. Pretty well rendering the flag moot.
Either the docs need to be updated to specify that case or the code should be changed to handle the serialization as it used to work in D6.