The only thing that has changed that has caused your issue is the iChat client software. So your issue lies there, I would seek assistance from an Apple/iChat type forum that are familar with it’s configuration and what changed between the two versions to get an idea on how to fix it. If it is affecting OpenFire it may be affecting other Jabber/XMPP Servers.
I have downloaded version 3.7.0, saved the 3.6.4 config files and copied 3.7.0 over 3.6.4.
Restoring the saved conf-files into the 3.7.0 config folder and starting openfire.
Under the “Plugins” menu load the “ClientControl” which will load nicely. Under the server menu you will find now a “Client Management” tab which you should select.
Select the “Permitted Clients” submenu. Either you select the radio button “All Clients” which will allow anybody to connect to your openfire server or “Add Other Clients” to add the “imagent” keyword and select the “Specify Client(s)” radio button.
You should be now able to connect with iChat for Mac OS X Lion to Openfire again !