API for getting members of a group doesn't fetch from smart groups

petednz reported to me that civicrm_api('group_contact', 'get') with group ID passed as a param was not fetching "smart" members of a group, only static ones.
Not sure what's up with this, but the API must be calling some pretty crusty method since the rest of CiviCRM is so nicely smart-group aware.
I'm happy to work on this, but could use a hint about where to look for the correct function to retrieve members of a group.