The activations are per system, i know that. I'm not sure if it's per game per system (like if you got Ratchet + Clank PSP could you put it on 2 psps AND 2 Vitas? <--that I don't know).
Actually, just looked up the ToS buying a PSone game (in this case it's chrono trigger)
"One-time liscense fee for downloads on 2 console systems and 2 portable systems that are associated with the account."
So in this case it sounds like you could buy chrono trigger and put it on two ps3s, a PSP and a vita (or 2 psp/ 2 vitas, but not both only 2 total). This is in a parallel universe where the vita can play PSone games.
So yeah, anything you buy that can be on all 3 platforms that use the PSN you actually have 4 activations, it's just split 2/2.
And too answer your bigger question, you can put your user account on as many Playstation devices that you want (in terms of being signed in to play online (assuming it doesn't need a pass), to get trophies, or whatever. It's just that as far as purchases go PS3 only can only be put on two (and it's the same two for all games, you can't buy Game X and put it on PS3-1 and PS3-2 and then Game Y and put it on PS3-1 and PS3-4). And any mobile purchases can only be put on two portables. And anything that can work on both (right now would be PSone games and minis) can be put on two console and two portables. But if you have a third playstation you can still sign in and play disc based games, you just won't be able to do anything with paid downloads.
I hope this helps. I think i was confusing.
Edited by famousmortimer, 20 May 2012 - 02:48 AM.