It's probably similar-ish to the way the PS3/Vita/PSP is set up now. There's no extra limitations made public at this point. So, generally speaking, the status quo is maintained.
It hasn't been confirmed, but their wording made it seem like they were avoiding the question. I suspect we'll have it clarified sometime tomorrow whether or not PS+ is necessary for multiplayer. Sooner if it's not. If we don't hear anything by the end of the week, I'd say it's a given.
Never had any stability problems with PSN. Then again I don't live in buttfuck Australia.
EDIT-
The wording was something along the lines of "with plus you get this this and this and also access to all of our multiplayer services. With regular PSN users can still utilize all of our network functions and enjoy their single player games uninterrupted".
It didn't say "you have to have PS+ to play online" but it didn't say "PSN will have free online play". It could just be lousy wording or something, but it may be hinting that they're going with tiered online the way Microsoft did.