I've only experimented with journaling in Exchange 2003, so I do not have too much experience with this. However, curiosity was killing me. In 2003 environment, journaling is enabled per mailbox store basis. The similar functionality is available in 2007 using standard Journaling:
Enable Journaling: Set-MailboxDatabase
Disable Journaling: Set-MailboxDatabase
If you want to find out more about premium Journaling, which requires enterprise CALs, then waltz over here.