Quick note:
using LDAP’s default port 389 with a Base DN of the parent Active Directory domain only shows objects from the parent domain. Changing the port to 3268 but keeping the same Base DN allows LDAP access to users from the child AD domain.
So, finally got the Openfire Jabber server to see all users from both domains.
Found via: http://www.igniterealtime.org/community/message/155746
For further reading: http://technet.microsoft.com/en-ca/library/cc978012.aspx
Kind of a lame first post, but hey, gotta start somewhere!