GT, he's GoPhone+data plan just like you. He just didn't call in and ask. Sounds like they told you straight, as I've yet to hear of a contract user getting such a notice -- I think on contract, SMS, and MMS and IM (if they were supported by iPhone), are all included in the plans already. Thus nothing has changed for us.
Oh well, here's to new firmware Q1 2009. Seriously, if I had to bet, I'd say next Tuesday, since all iPhone updates, all two of them anyway, so far have been on Tuesdays, later afternoon or evening. And next Tuesday is the last Tuesday that fits the "later this month" statement.