I was reading the change logs for Wildfire 2.5.X when I stumbled across the below thread in the WF forums. Appartantly WF has supported XMPP-compliant invisibility via JEP-0126: Invisibility since V2.5.0.
In the below linked thread, user dombiak_gaston mentions that privacy lists were being incorporated into Smack, and we could therefore look forward to a release of Spark that will support invisibility with Wildfire in an XMPP-compliant fashon. Comments?
http://www.jabber.org/jeps/jep-0126.html
http://www.jivesoftware.org/community/thread.jspa?messageID=112890#112890