Keep in mind that modes are case sensitive, +i is different from +I, I can't recall ever seeing a network straying from the standard meanings for the biklmnopstv channel modes.
~ Edit ~
As long as it's been around, +I has always meant 'Invite Exempt' meaning a user can join an invite-only (+i) channel. That and +e (ban exempt) have remained pretty much standard across all networks that support them as far as I have witnessed, so Khaled added support for those in Channel Central a few years back. Whether or not they have remained standard only because mIRC supports them as Invite Exempt and Ban Exempt, I don't know.
