Somehow I missed this, but apparently Sony will let the publisher decide if their free-to-play games will require a PS Plus subscription to play online.
Most likely F2P games will simply work with or without PS Plus. The question is if a game does require a monthly subscription, how will that tie in with PS Plus, and will such subscriptions be cheaper for PS Plus subscribers.
Most likely if a game uses a subscription model it wouldn't require PS Plus, but you won't get any discounts if you have PS Plus. We haven't seen any game yet that suggests a mandatory subscription model yet though (only F2P and micro transaction models). However with many subscription based MMORPG games moving to a F2P (with micro transaction/in-game store) model we may be seeing the end of mandatory subscriptions in the future.