You can use ADAM to combine your AD domains (realms) so that you only need one Openfire server. This is in reference to http://www.igniterealtime.org/issues/browse/JM-919
An Update to this. (JUNE 2012)
ADAM by default does not allow plaintext binds over 389. You need to configure your server with an SSL cert and use port 636.
If you are just in a sandbox environment you can turnoff the no plaintext over 389 by making the following settings under msDS-Other-Settings in your ADAM Configuration using ADAM-ADSI Edit:
Just be away though that all passwords are now flowing plaintext and are subject to sniffing.